Symfony dumped all symfony1 users up s* creek when Symfony2 came out; Zend framework 2 is an entirely different beast to ZF1...
Are these mature frameworks any more stable than the new ones, or are they new frameworks, phoenix-like in assuming the name of the old one?
On the other hand, major version releases are when you make such dramatic departures from what's come before. At least in ZF, they got a lot a lot of things more-right.
I moved a (pretty complex, line-of-business transactional-type) project from ZF1 -> ZF2, and I'm happy I did. Some things changed a lot (dispatch, routing, forms), while other things were very easy to translate (caching, logging, etc).
In that case, most of the business logic was built around Doctrine2, encapsulated nicely into service classes, and therefore fairly framework-agnostic.
I'm glad that transitioning your project from ZF1 to ZF2 was worth the effort and relatively easy. I have a number of clients still on symfony 1.4 because although unsupported and a 'dead' system, without a straightforward migration path to Symfony2 it doesn't make economic sense for them to do so - not when the whole web system (CMS, order management, the rest) is written using symfony 1.4 and Doctrine 1.2. Until it stops working, as it's going to be a new build either way.
I think that kind of framework dead-ending is fairly irresponsible; certainly some of my clients thought I was irresponsible to have chosen Symfony when their 1-2 year old systems needed to be rewritten...