No need to convert to polygon.
In your workflow did you ensure that the spatial extents, cell size and snap raster were all the same (some are within your control)
Did you do any filtering of either the input or output data? (the methodology should be discussed)
If you want to remove small fiddly bits, you can use filtering (discuss types used) or the shrink/expand method (discuss differences)
Where are your histograms?
You have a weird time step, what happened to the intervening years? making statements about time differences fails to draw attention that two of the differences are for 4 or 5 year differences and 1 is for a single year difference. The visual implication/inference needs discussion.
Normalize? maybe, did you look at this. And are you refering to normalize within a year or between time steps?
Regiongroup can be used to determine the number of unique groupings since a raster class is in essense a multipart 'shape' and regiongroup does the same function as multiparttosinglepart in vector world
Converting to vector introduces a whole set of other issues, if you feel the need, don't generalize, keep the cell shape intact so that cell counts/areas should be the same.
Oh... any spatial variation that can't be explained solely by invasion landscape preference (ie elevation, spread vectors...ie how spread)
Thats all I can think of now.