I would think only new blooms would be affected.

i have two endless summer bloomstruck hydrangeas. One was in the ground and one in a large pot. The potted one has pink flowers

The one in the ground put out beautiful blue/purple flowers but was in too much sun and has 3 octopus armes, so I dug it up and put it in a pot. Im thinking it will put out pink blooms from the potting soil now.

what did you amend with?

AS takes time to become effective, typically a matter of some month, which is why it is suggested to apply it in late winter for the coming season's flowers. You can try some now, but I doubt it will have much impact this late in the season.

My rebloomers that have blooms do not show much change their existing lacecap blooms that much. I am only trying to maintain the coloring though and not trying to add to make them purpler or bluer though.

The new blooms, ie the 2nd flush from new growth that will show up later may show the tweaks better although I have not been able to compare them; need to take pics in order to reaaaaly tell.

So amend well in advance of the flower buds from new growth even forming. My blooms from old growth have been pollinated and I may try at this point now to see what happens.

What do you amend with? Is it something you can just pick up at the garden center?