Hi,
From 8.3 on memory requirements changes (512MB) for ASA5505. Also, there are NAT changes. Check the below link to get some idea. Search in the forum , you will lot of postings with ref to similar upgrade. If you have a spare unit, I suggest you test the configuration (new) on that before move to production.
https://supportforums.cisco.com/docs/DOC-12690
hth
MS