Yet another shell way (showing there's always more than one way to bugger around with this stuff...):
$ echo 1.2.3 | ( IFS=".$IFS" ; read a b c && echo $a.$b.$((c + 1)) )
1.2.4
So, we can do:
$ x=1.2.3
$ y=`echo $x | ( IFS=".$IFS" ; read a b c && echo $a.$b.$((c + 1)) )`
$ echo $y
1.2.4