The best workaround is using a save game manager like SaveDataFiler to backup a save unencrypted. This allows transferring that save to the game once you downgrade (by using SaveDataFiler to transfer the save back to the game). However this requires an exploitable console and CFW of some kind like rxTools/Pasta/CakeFW/Reinand or a CFW with DRM like Gateway.
Also, as far as I'm aware, eShop games do not use any specific kind of encryption and relies only on the encryption on the SD card provided by the movable.sed seed. So those kind of saves don't use new encryption since the new save encryption is related to cartridge based games only. Same goes for extData like Battle Videos for Pokemon X/Y or ORAS.
Saves for eShop games will still work after you downgrade provided the movable.sed wasn't reset. (aka this usually would occur if you did a system format) of coarse the games of choice have to still boot if you are booting them from a 4.x sysnand (as newer games require newer FW SDK to run). But if it's emunand, the games will work unaffected and the saves will still work.
Also you said you can't downgrade because you're on 8.x? Why is that exactly? Are you using a 2DS or a n3DS?. If it's a regular o3DS or o3DS XL, you can still downgrade it. Just use Gateway's menu and the matching downgrade pack for your console.
Technically you can force downgrade a 2DS using DevMenu 2x and the 4.x system CIAs in Gateway mode. But 4.x doesn't behave correctly on 2DS and I hear the top screen becomes unusable. Or the bottom screen. I can't recall. It doesn't brick the console provided you can get back into the browser to restore a nand backup.
(had you been unfortunate enough to do this with wifi turned off, you have no way of turning it back on if you downgrade to 4.x with it off.)
Definitely not something you should try. And yes trying to do a system format on a 2DS that has been downgraded to 4.x or one that's had it's System Settings app downgrade to the 4.x version will be soft-bricked because 4.x system settings doesn't know it's a 2DS and tries to get you to do the 3D demo which is impossible to complete on a 2DS. The only way out of that situation is to use the recovery menu to update to latest fw or use a nand backup if you have a nand mod.
It is impossible to downgrade a n3DS because several important system apps that are specific to the n3DS and required to boot the console are not available in versions below 8.x. This is because most of said firmware files use new title ids and the bootrom expects them to have that title id. That and it would be like trying to run Windows 98 on a Macintosh...It's not going to end well.
If it's a 2DS or n3DS, your SOL. There is no way to convert old encryption method saves to the new encryption on newer systems and there likely never will be. Best solution is to get your hands on a 4.x system and use savedatafiler to decrypt the saves. This will allow you to transfer them back to the games on your newer console that is stuck on the higher firmware. Since you can use Gateway on your current console, your not completely screwed.
You can try to find someone who has a 4.x system to do it for you if you can't afford to get another console. I could help you, but I had to pawn my 3DS a little while back. That and you don't yet have enough posts to access the trade forum. Once you do have enough posts you can visit the trade forum and post a thread in the buying section seeking someone to do this. (I believe you need at least 100 posts. And don't spam to get to that count. The mods will ban you if they find that you inflated your post count to get access to the trade forum.
)
I'm sure paying shipping and a little extra to have this done for you would be cheaper then out right buying a second console to do this your self. Especially since you likely wouldn't have a use for the console once you retrieved your saves. It all just depends on how important the saves are to you. Because there will be no way to avoid spending money to recover them. There is no software solution for this that doesn't require a second console that can be downgraded.
Also if the user also owns a Gateway, (I guess a Sky3DS might also work) you would save money by not having to ship the games to the person. Instead you give them the rom and the save (if it's a card2 game, the rom contains the save) and they can just put that copy on their Gateway card and extract the save for you.
EDIT: I forgot that I no longer own a Gateway (and I'm not stupid enough to buy a Sky3DS.
). It's unlikely I could help you myself even if I had my 3DS on me. I would need a slot-1 flascart of some kind to store the rom so I can extra the save. So unfortunately there's nothing much I can do to help you myself. I could still help if you owned physical copies of the games. But not if you only had roms of them.
