You might also have a check-up lesson to see if the "deterioration in long iron play" is due to swing flaw or equipment mismatch.
Then, during fitting, get a gap analysis of your irons. The gap analysis determines the point in your iron set at which the next longer iron goes the same distance - or maybe less distance - than the previous iron.
For instance, if you hit your 5-iron 170 yards, and your 4-iron 178 yards, and your 3-iron 170 yards with inconsistent contact, chances are the 4 iron is the longest club you should carry. (Unless, of course, you embark on a focused learning program to master the 3 iron).
The fitter would then match you with hybrids to fill the gap between 4i and fairway woods.
In many of the blended hybrid-iron sets, you can replace the longer irons one-for-one with hybrids made for the set. If you bring in hybrids not fitted to the iron model, then you need to experiment some to find the right hybrid mix.
Also, caution..... not all hybrids are easy for a given golfer to hit. Test before you buy, preferable on course. A friend of mine got 3H, 4H, and 5H matching his irons, and can't hit the hybrids very well. Irons and FWs are OK, but those particular hybrids don't work for him. He said he may add a 5 iron, and pick up a couple of "outside" hybrids for his bag.
I was the same way. My first two tries with hybrids were discouraging. The 3H was OK out of the rough, but tended to spray badly off of tee and fairway. It took me over a year before I found a model that worked.