What are some ways to transition from additive to multiplicative(or vice-versa) changes in time-series quickly?
The problem with your request is that both ways are always possible. For any pair of values x and new_x you can choose to represent the increase either as an addition or a multiplication:

additive increment: new_x -x
multiplicative coefficient: new_x / x

So you need a criterion to decide which one to choose, for instance a threshold: if x is lower than 100, use addition otherwise use multiplication.
Additionally if any kind of change can happen at anytime it's completely arbitrary, there's no way to represent this in a systematic way.