Handling Merging and Change to item IDs

I have this case that I am not sure how to model.
I have a product dimension and a fact table (invoices line transactions). In the source system some times they merge part numbers or items id into new ones, or change some items id to different, in our data warehouse every time that this happens we update our transaction table. I am not sure how to model that into dimensional modeling, I will need to update the fact table but I don’t think this is the best practice, also I don’t have any log table that I can rely to keep tracking of the changes only that the source table that contains the transactions at the line level.