In general I do what JudoP said: drop from (5x3) to (5x2) or etc. or pull the dropped string into the core as joe_meadmaker described. Going from e.g. (9x1) to (8x1) can work also if you have thin strings and many of them.
With natural material / fibers you are much more flexible. With fibers, I sometimes work by thickness, not by number of elements per braid. You can easily add/reduce fibers.
Natural string often consists of several threads twisted together. Consider removing them one by one, before dropping the whole string.
Pulling strings into a core works very well. I never had one becoming loose. The remaining braid, when pulled, will pinch the core into place.
By the way, this works also for the normal "fishtail" braid. Lakeslinger once posted a nice picture a while ago:
http://slinging.org/forum/YaBB.pl?num=1389365336/2I do not recommend tapering with thick synthetic "rope". It usually looks quite ugly. But you might consider gutting the rope, if it is a core/mantle one like paracord. You will have to sew/glue, and the transitions are quite obvious. Better start right away with just one string or drop all except one close to the pouch. One paracord rope is more than strong enough
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Changing the braid is a possibility, e.g. from 8 strand square to 6 round with the rest in the core, then cut out the core one by one. But it is quite difficult to braid and make look nice. I don't recommend it.