Hello SK,
It depends of the settings attached to the release code :
It is possible for instance to avoid resetting the release strategy if the value is changed within a predefined interval using the changeability key 4 and setting up a percentage.
Also, it is possible to use the key 2, in this case there will not be any new strategy for the change.
Otherwise, the standard behavior is to start a new strategy. There are several reasons for this, one aspect is that the release codes are assigned via authorization another aspect is the workflow integration...
Overall, to have complex rules like "do not reset the release strategy if the value is decreased", it requires some development. It is possible to link characteristics to function modules and so on .. and much more !
Have a look at 493900 - FAQ: Release Strategy
Regarding enhancements, there are resources on scn like Purchase Order Release using ABAP Classes - ABAP Development - SCN Wiki
Cheers,
Alexandre