Replace PNG, but retain transitions/movements

Hi people, I've done a fairly comprehensive edit of some footage which shows a crowd of people gathered together and I needed to highlight individuals as they moved around.  I designed various coloured arrows in PS and saved them as PNGs, imported these to PP dropped them on the timeline, corrected the size and then moved it around above the individuals coverned on a frame by frame basis, so it's nice and smooth with the footage.  My client suddenly wants to change two of the colours we've used and so I need to establish a way of just replacing for example the red arrow with a yellow one, but retain all the key frames/edits previously set up.  I know this can be done in very basic editing software like Correl by just right clicking on the image and select replace with.... so surely there must be a way in PP. 

Hope someone can help.

TIA, Luke