Hey there!
As far as I’m aware you can’t migrate assets backwards with UE4. If your product is for 4.20 you can just submit it through there, the requirement for supporting the last two engine versions isn’t for new products but for older ones. i.e. If I had a product that was submitted with 4.18, with 4.20 out I still need to have 4.18 and 4.19 supported. Sellers submit content with the latest version being the only compatible all the time. The only thing is some customers may not be using the latest, so your sales may not be as high as it would be if you supported previous versions out of the box. What you’ll want to do going ahead is develop your product on the earliest version you want to support, and then once finished convert them to the later versions for compatibility. If nothing changes you’re good, if some things do you can make a unique version for that engine version and submit it along with your product. Me personally, I’ve been developing on 4.16 for the last year or so, and then converting them to the latest. That way when I submit new products it will be compatible with 4.16 through the latest. Good luck! =)