Performing upgrade from a live site on a localhost

Hey guys,

I am looking to upgrade my live webshop from 1.5.6 to 2.3
So far i understand the upgrade process on the live website, but i don't want to much downtime.
I have made some custom adjustments and have different add-ons installed of which a few become obsolete and are not needed anymore with the newer version, others need to be updated aswel too.
For my current site i completely redid the default theme and this is what i'd like to do with my upgraded also.

To keep downtime to a minimum i'd like to perform the upgrade on a localhost first and than perform it on my live site.
This way i can install all the needed add-ons, change the theme etc while my live site is still up and running without changes.
Now comes the difficult part for me... i have never copied my live site to a localhost yet and got to work properly.
I tried this several times but i am affraid al though i have read different how-to's i am doing something wrong.

So i have a few questions:
Before i go do this
- When i backup my live site to a localhost and than perform the upgrade and swap that back to my live site, will my stock count be from the time i upgrade the live site, or from the moment i downloaded the backup to the localhost?

About setting up the localhost
- Would i need a fresh install at my localhost first, before copying my live site to the localhost?
- When i copy my live site files and folders to my localhost, do i perform an install of some kind?
- How do i get/make the appropriate config files for my localhost if i do not use a fresh install?

About copying the upgraded localhost files to the live site again
- Do i need to install the upgrade on my live site first and than copy the edited files from my localhost into the live site folders ? Or do i copy the upgraded files from my localhost directly into my live site folder and than perform the /install command again to initiate an upgrade?
- Will i have to manually edit the config files to match the newer version, or will it automatically be updated?

Thank you so much for your time, hope you are willing to give me (some of) the answers.
Kind regards, Anton

To keep it simple:

1. Never touch a running livesite!
2. Whenever possible, make backups, backups and backups
3. Never make a database backup with the shop internal backup tool, use instead phpMyAdmin (or other) - reason: you need the table structure
4. Either install the new version local or in a subfolder of your current server (the where your livesite is hosted)
5. After installed the new version, convert those database tables you need (basically customers, categories, products and orders)
6. Install the theme
7. Now go ahead and test as much you can this new installation
8. If you have made all this locale, transfer everything (files and database) to your server where your livesite is running into a SUBFOLDER
9. Adopt the config files
10. Now again: test and test
11. If everything is working as expected, either point to this new folder (via your cpanel) or move the files into the livesite folder >> if overriding the exiting livesite you will have a downtime of a few minutes, if pointing to the new folder you will have no interruption
12. Bevore you activate the new shop, add the new customer and order table entries to the new database (never use the original)

Doing so, you will have NO downtime (except a few seconds).
And you will have time to test while the liveshop is running and running ..

@OSWorX
Thanks for your reply and your extensive explanation!
If i perform the upgrade like you described, won't i lose all extension/module settings, info pages, site settings etc?
I don't really feel like manually adding all my information pages, settings etc to the new install again.

The text (information) tabes have to also converted - forgot them.
Settings: they will be created new, no need to convert them (and because the use another format).

About the extensions and module settings: because of the new format of the settings table, it would be better to define them new (and is much than faster doing manual).

Another thing is, you have to check if the extension and modules are compatible with the new version (which almost none really is).

Ah ok i thought so.
Well, was already about half way through updating a copy of my existing site, when i read and replied to your post.
Did finish but didn't know about the compatibilityproblems with 2.2 and earlier extensions in 2.3.
Thought 2.x extensions would work on 2.3 but guess i was wrong.
I have to try and fix this issue as most off the extension i need to have are 2.2 compatible and do not yet have a 2.3 version.
So far i found i have to delete some obsolete folders and install a vqmod fix to add compatibility for 2.2 extension.
If that wont work i will have to install some older version like 2.1 or 2.2, as both my parcel service and my payment provider only have 2.2 compatible extension, without these i am getting nowhere.
I really dont feel like upgrading everything manually, it is way to much work in time i do not have.

Updating extensions from 2.2.x (2.1.x) to 2.3.x is not that much work as some might think.
If you need help with them, you can contact me at any time.

So it is possible to get a 2.2 extension to work in 2.3 without having to wait for the developer?
Was googling for information to do so but can't seem to find anything helpfull.
I got my site working and have been cleaning out some obsolete folders etc.
Noticed when installing 2.2 extension they create back the folder i deleted as obsolete.
Eventough i can install them succesfully, get to the settings etc the modules dont seem to work properly.
When saving them i get an error like:
"Notice: Indirect modification of overloaded property ..... has no effect in /.../... on line .."
Even than the settings are saved and seem to work, but when i get to the frontend they are not working or even causing faults like empty buttons etc.

So yeah... if you are able to tell me how to get 2.2 extension to work in 2.3 it would be appreciated.
In that case i might keep on to 2.3 otherwise i will start downgrading to 2.2 or 2.1 even.