what happens to mods after core upgrade?

I have a Multivendor cart now on joomla and Virtuemart. I am considering switching to Zencart. This is my first question and I have a lot of questions! ;-)

I see Zencart has a lot of addons and mod packages that can be installed. I'd like to install:

• Multivendor
• custom designed template
• affiliate
• customer reviews of products
• ugc (user generated content, like pics and video)
• fast checkout
• PP and GC
• Multi-lingual
  ...Etc.

The question: When an update to the core package is released and I install it, *how often * do the add ons become incompatible? And typically, how long until the developers of the add ons release the fix? Please provide personal experiences.

Thanks!
Leafgreen :D

Here's the reason I ask. Here's the post I made on another cart forum, also researching:

I see many threads about customers asking questions about upgrading from 4.1 to 4.4, etc. Typically, why wouldn't customers upgrade every time a new release comes out?Answer:> Because it is very difficult and time consuming to upgrade a customized store. I will have to redo all these small mods in the new upgraded store if I do an upgrade.
my reply: > But I sure wish there was some coding architecture with objects or includes that prevented co-dependence... The situation that everyone is in with upgrades to modded carts seems like self-inflicted torture.
Does this apply to Zencart, or did you guys figure out a better way?

You still need to merge any core files that have been modified by a mod.

You also need to upgrade any template files which essentially are modified clones of the core installs.

Check and merge every mod you have installed.

There are tutorials and a wiki, links in the header of this page, study them carefully.

Whether addons become incompatible depends entirely on whether they're doing things directly related to changes made in the updated core code.

@frank thank you kindly for your reply, but respectfully I was not asking about how to upgrade.

@drbyte and everyone: still seeking answers to the how often and how long questions originally posted. TIA.

Add ons are made by people out of the goodness of their heart ... you know ... for giggles and grins ...

That means they may or may not update an add on with a new release of Zen Cart as they don't make any money out of it and are not responsible for the code ...

So, when you upgrade your Zen Cart Code ... the add on module may or may not work ... and if it does not work ... the author may or may not upgrade their add on for that new release ...

Usually, someone will come along and upgrade an add on that fails for some reason or another between one release or another ... or, there is usually a Contribution Thread with an add on that you can ask questions or ask for help if something fails ...

But, as far as your site goes, you still need to merge files from one version to the next if the add on touched original core files ...