Hi Jay, You can use any hybrid shaft in any head - the difference in your 17 depends on the shaft, but in general, it is 1" longer and your 17° was not tipped and the 21 was probably tipped 1".

What do you mean by tipped?

When we receive a shaft from the OEM vendors, it comes at a raw length.  We will cut the tip end (tip it) for flex and different lofts and cut the butt end to length.  On a 17° hybrid, we don't cut the tip end and the final length is 41", on the 21° hybrid, we will cut the tip end 1" (depending on shaft) and the final length is 40".  That is our standard installation.

Like they say, it is do-able.  The 17* shaft will add ~6 swingweight points and may soften the effective flex slightly.  The other compensation is flattening the lie angle for the longer shaft (if you use A1 on the 21* shaft you would use B1 for the 17* shaft). 

I'm doing the opposite with my 19* fairway.   I butt cut the shaft to 41" and put on a Winn Lite Dri Tac grip to bring the swingweight back up.  I have the setting on A3 so it almost swings like a 17H but much easier to hit.

Same thing with my 21H.  It is only 1/2" longer than my 27H and the lie angle is .25* flatter.  Hits perfect.