Inquiry regarding general theme support of WP core upgrades

[catchthis]

Hello,
I currently use StudioPress for some of my clients and am looking at it for another client but I have a basic question first. The client in question is currently on a Joomla platform but due to the retirement of pre-2.5 versions I have been informed that the template they use will no longer function if I upgrade.

As it is going to require some work and I prefer WP, I'm planning to do a rebuild on WordPress this time and wanted to know if you can tell me what sort of life span there is on your templates with regard to WP core upgrades? I would hate to be hitting the client up with this issue again anytime soon.

Thank you,
Shayne