Damianik,
If you are assigned to the G5 upon joining then your "seat lock" is only for 12 months after line training. Given that upgrades are at least 2 years away, this will not put you at any disadvantage. If you bid to change to the G5 from another type then you are locked in for longer ( 4 or 5 years I think) which is what Mr Falconbis is referring to.
Its all in the ops manual and will be explained to you (as it was to me) at your INDOC.