There are several ideas. They may be imlemented separately.

1. Make troops (that are placed in a colony) automatically upgraded attack and defence strength with respect to current tech level.
The attack and defence strength should be increased to level of newly-recruited troops, if it is less that that.
I.e. this should be in most cases same as to disband the troop and recruit again - but without manual intervention.
(This is probably the easiest thing to imlement. I propose it to be done in next patch.)

2. The troop species will be automatically changed if new species make better troops. (may be configured in Empire Policy)
Readiness should probably be temporarily reduced.

3. Also, if there exists a required planetary facility:
  1) Infantry should be upgraded to clone troops in following cases:
   - if upkeep is lower or same. (may be configured in Empire Policy)
   - if upkeep is bigger. (may be configured in Empire Policy)
  (Upkeep of clone troops might be different due to species-specific upkeep bonuses)

  2) Infantry should be upgraded to elite infantry, if cannot upgrade to clone troops. (may be configured in Empire Policy)

4. Items 2, 3 - can be done manually by user in Troops window

5. The troop globally used for cloning may be selected by user. It is especially good if you have troop that has bonus for reduced upkeep.