SPD and Transformation Attack

[Blue]

I think I can extrapolate an answer, but I thought I'd ask anyway.

FREd states that you can only change your SPD between turns. If I had a Transformation attack (Say a Potion) that turned someone into a creature with a higher SPD or that simply boosted the individual's SPD by a few points, would that SPD change not take effect until the next turn?

Thanks for your time.

[Steve Long]

The "Changing SPD" rules on 5E 232-33 apply, though you can use the optional version if you can convince your GM.