Answer: You're correct, cutting them back now will sacrifice next year's blossoms and, depending upon how far you cut them back, may keep them from blooming for several years. The rule of thumb is to prune them back when they have finished flowering, and to remove no more than one-third of the plant in any pruning session. So, if they are very overgrown, it might take 2-3 years to reduce their sizes. In any case, I would not cut them down to ground level.
Best wishes with your landscape!
Q&A Library Searching Tips