like the previous poster says,make sure you have the same major/minor version of OS running on both firewalls.
Also, note that PIX doesn't support ssl vpn's with any OS.
I've never had to 'downgrade' from ASA to PIX, but when upgrading to ASA from PIX, i've found the safest way is to copy/paste the existing config section by section into the new device, that way you can account for any problems easily.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
[toc:faq]Introduction:This document describes details on how NAT-T
works.Background:ESP encrypts all critical information, encapsulating
the entire inner TCP/UDP datagram within an ESP header. ESP is an IP
protocol in the same sense that TCP and UDP are I...