The word “elect” may be a bit misleading there, it’s a one time action for the currently held maturity only. But the warning is with good intention, merging the currently held maturity isn’t reversible and will lock up the ICP minted by it at that moment.
(Note the merge maturity command also allows you to merge just a percentage of the maturity held and leave the rest untouched).
It’s likely to change, there’s a related discussion on the handling of maturity etc here. The need to spawn a new neuron and dissolve could be removed in the course of all this.