Nothing will replace it.
Some admiral type explained it to us recently thus:
Imagine the Navy as like a motorway with several lanes. The biggest lane, the outside lane, is the regular service. The next lane would be the reserves, with other slower lanes for people on sabbaticals, career breaks, fleet reserve etc.
The idea is to make it easier for individuals to increase or reduce their committment to the service as required. (ie. by changing lanes.)
Therefore, it will be easier to utilise reservists to fill regular billets, and for regulars to drop in to the reserve if they wanted to go to university for example.