I am in the process of up grading my 603 to a 606, I am using the 603 ip cause my car don't have any of the electronics ! can I remove the 606 plungers and switch them to the 603 ip ???? thanks J

You can't, but an injection shop can.

Will there be a dramatic differance Between the 6mm compared to the 5.5 ?

Around 25-35hp.

only in case there is a + in air flow to be able to burn the extra fuel.

Having that done in an injection shop at normal labour rates I fear it does not make sense (unless the 606 ip is damaged on the other end and the guy in the shop is a good friend).
Try to get a set of 7 mm elements from Myna, have them installed and sell the 606 ip complete.

Tom

Thats kind of obvious.