I think the only issue (well, at least the one major issue I'm familiar with) would come up if you have scripts in your 5.00503 code base that try to do things with Unicode or other wide-character data. You'll want to redo those scripts to use the much better wide-character support in 5.8.

Related to this, I've heard about (but not experienced personally) problems that arise if you are on a particular linux distro (Red Hat 8 for sure, maybe 9 too but I'm not sure) with a particular user environment involving the use of a "utf8 aware" locale, and you have scripts that handle any sort of unformatted-binary or wide-character data. Basically, given the "right" set of triggering conditions, a script that worked in 5.005 may fail in 5.8.0 with error reports about "malformed utf8 data", despite the fact that the script and data have nothing whatever to do with utf8.

If you use the PM "Super Search" for "utf8 error", you might get some tastes of the details (they are a bit arcane, and there are simple patches to make older scripts behave as intended, if this turns out to be a problem). Apart from that, I have found a few items in the Unicode support (a couple obscure regex problems and some of the conversions to/from non-unicode character sets) that aren't quite right in 5.8.0, and I'm eager to see 5.8.1 come out -- but this all has to do with stuff that 5.005 could never have touched in the first place.

In general, things that worked in 5.005 work fine in 5.8, and the transition does not involve complications. Of course, you'll be "re-installing" modules for 5.8 if they involve compilation -- don't just re-use the modules that were built for 5.005.