When you trade with a villager, swap its workstation to another villager, give it a different workstation then remove the original workstation completely the original villager changes profession to the remaining workstation, with all of its levels still kept (e.g. lvl 4 blacksmith to lvl 4 weaponsmith)
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We're tracking this issue in MCPE-117852, so this ticket is being resolved and linked as a duplicate.
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