I think the safest option is to conduct a 5 round draft this year, change your league settings to match the amount of draft rounds your league rules state, and then things will go back to normal next season. If you allow GM tweaking with spring training then you pretty much are stuck with a choice of either 5 rounds this year or not allowing GM tweaks in spring training.
While you are changing your league settings, may as well make sure your other league rules have been set up properly too

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