They are synonyms. Retrogressive comes from adding retro - (Latin for "back") to the ending of "progressive" (from the Latin progressus , "having walked forward" or "an advance"), while regressive comes from the Latin regressus for "having walked back". Regressive is the older word.
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After looking around I'm seeing regress as, 2-steps forward and 1-step-back along a range, where retrogressive as, 2-steps forward and back on all fours shift to a prior category; crawling vs. walking.