I’m upgrading from a rather old MySQL, this is basically a button press and I suppose I should dump the data in case something goes awry but I cannot see why my boring EO Model would have any trouble using an updated version even if the jump is — well, at least like…I dunno - ten years?
You mean you're already on RDS and just need to upgrade? I've certainly never encountered any problems, though I'm surprised you could get a decade behind. RDS is more aggressive than that with PostgreSQL, at least, forcing major version updates from time to time.