Yes, it's not as simple as pressing an Upgrade button.
The behaviour of XQuery functions and even "allowed syntax"
changes between releases.
There are incompatibilities between various minor versions
of MarkLogic Server, let alone major versions and in most
cases you cannot simply downgrade MarkLogic Server if you
wish to rollback from an unsuccessful upgrade.
MarkLogic Server, although written very well, is built by
humans and so are your systems; the more complex a system
is, the more likely there will be potential issues, which is
why an upgrade plan and a rigorous user acceptance testing
procedure is very important.
If you don't upgrade your MarkLogic instance in time and the version
you're on has reached it's "end of life" phase. MarkLogic
may potentially refuse to provide you with support, if your server
then encounters a major bug you may not be in the best position.