If the history is all on USD entities, then I don't believe changing the rule type will matter. If the history is on foreign currency entities and the rates entered are all 1, then again, there's no difference since both the average and closing rates are the same. If you were to go back in time and reconsolidate/retranslate, the those values would be the same since they're all on USD entities or the rate stayed the same at 1. The only time you would see a difference in translation for average and closing is if those rates are different from each other by time.
The beginning balance piece will be your challenge since at prior year M12 it translated at 1 which is not accurate. You'll need to find out from the customer how they want to handle that piece. You could write a rule to translate begbal in CY M1 but then in USD, it won't tie to PY M12. Does that make sense? Sounds confusing to me when I write it.
Assuming we're talking about Dev, the 2017M12 balances will be translated the same regardless of rule type because there's not prior period data to translate. OS will view the entire balance as activity for that time period. So you should be fine changing the rule type on that cube.
