Yes. You will not be locked from playing a spec or another base class entirely. Assuming you have you classes leveled:

Today you're playing DRK on Gladiator, maybe later you want to switch to PLD, wait you changed your mind and want to play Lancer and work on DRG.

You can do these things. That is a staple of the franchise and my favorite thing about FFXI. I do not like games that force players into making new characters to play a different class.