Re: Roman Soldier Retirement
There is little good information for the conditions of service in the Roman army after the 3rd Century AD, but I'll answer as best I can.
In the early Empire, a soldier received a final, huge monetary payment and a plot of land, as his final rewards for his 25 years of service. I honestly don't know - and I doubt if anyone does know - if these were still given to a soldier at the end of his service c. 400 AD.
There was probably some kind of ceremony for veterans; very likely they were honored in some way in front of the legion; if they were coming from an elite unit perhaps they were even honored in front of the emperor.
There is lots of evidence from tombstones and other inscriptions, that Roman soldiers did often stay on after their 25 years. Whether they did this voluntarily or were forced, is unknown. Some soldiers (e.g. Aurelius Maius of the II Parthica, buried at Apamea c. 230 AD) stayed on to help train recruits, and probably served in this capacity until they were dead or senile.
In the Republic and early Empire, any man with military training and experience was liable to be recalled during a time of crisis, and presumably this was the case in the later Empire as well - when crisises were more common!
There is, as I said, very little evidence on the conditions of service for common soldiers in the later Empire, so unfortunately there's not much else I can tell you. I hope this post helps you somewhat.