If they bloom only once then there is a good chance that you may get few or no blooms if you cut them back by a third. However, you could temporarily enjoy the blooms and prune a few weeks after they bloom. I am assuming that you do not HAVE TO prune now.

Hydrangeas that are old and unpruned benefit from pruning the stems all the way to the ground (a few inches above the ground or so) in yearly 1/3 increments. In year one, you can prune the 1/3 longest stems. In year two, you can prune the second third longest stems. In year three, you can prune the remaining third.

You could also mix those two suggestions as well. Just remember that, for hydrangeas that bloom only once, new flower buds will develop in July-August. For example, prune only a few stems now and prune the rest in June-ish.

For lacecaps that rebloom (I can't remember if they go back 10 yrs), you could prune the existing stems by a third now and, later this year, you will get bloomage. Or use one of the suggestions given above.

Luis