Say that again? I think you're asking if it's possible to change cylinder heads without disturbing timing? If you don't already know the answer to that question you're probably better off having someone else change your heads!
I'm assuming you're talking about an older (non computer) sbc here.. In order to pull the heads, you have to remove the intake. In order to remove the intake, you have to remove the distributor, carb, throttle linkages, any accessories bolted to the heads etc. After removing all this, you're going to have to reset your timing, valve lash, etc.
If you decide to go ahead with this, get someone who has done it before or has built an engine at least. There are lots of good books on building engines that have all the info you need. You're basically taking the engine almost to short block so it's a pretty involved process.. Good luck!