With the Slackware 14.1 Upgrade comes X.org 1.14.
AMD dropped support for ATI HD2000, HD3000 & HD4000 Series in their "Legacy" driver*, they offer:

• Version 13.4 of 5/29/2013, still supports these cards (at least I guess so), but only works up to X server version 1.13
• Version 13.11, supports X.org server 1.14, but not the Radeon HD2000 (RV610,RV630,R600), HD3000 (RV610,RV620,RV635,RV670) and HD4000 (RV620,RV7x0) Series of graphics cards

* Note that at least some of the graphics cards in question are not even five years old.

How much would I need to deviate from stock slackware64 14.1 to downgrade the X.org X server to version 1.13?

1. Are there many other libraries or packages I would have to downgrade because they became incompatible with the change to 1.14?
2. Would I have to recompile all packages built against X?
3. What else to consider?

Thank You for reading,
bert

i would think that means using the opensource driver. dont be a fan of downgrading.

I have this solution already in place, yet it is quite a recognisable performance drop (the card is from the low end of the 4000 series).

When running the open source driver I strongly recommend to upgrade your kernel to version 3.12 and your Mesa to version 9.2, you will get performance improvements, dynamical power-management and hardware video decoding using VDPAU.
I run my HD3200 (sadly those don't have hardware video decoding support yet) this way because I didn't want to stick with the older kernel versions and ditched the proprietary driver for it. My HD6870 runs in the same configuration and works fine even with demanding games.
If you really want to use the proprietary driver, why not just stick with 14.0?

Right, I didn't even think about that....
Thanks, also for your other suggestion.
bert