DISCLAIMER:This guide won't be very useful right now, but as soon as 1.9 is out some might want to use it to keep using mods.

So, you will be asking yourself, "how will I use my beloved mods after the 64 bit update ('mod -killer')?"
The best solution I found was using both versions.
How?
Well, the 64-bit version will be the PlayStore one. But for mods you need the 32-bit one. So, you need to keep the old version saved somewhere.
To do this, use your file manager, go to storage/emulated/0/Android/data/com.jundroo.simpleplanes and save the mods folder somewhere (Google Drive, Dropbox, whatever)
Do the same for storage/emulated/0/Android/obb/com.jundroo.simpleplanes

This will back up the mods and the data needed for the 32-bit version.

Now open any program which allows you to create a backup apk of Simpleplanes. Save it.
When you need to use mods, put a 1 in the name of storage/emulated/0/Android/data/com.jundroo.simpleplanes folder and uninstall Simpleplanes.
Then install the old obb and mods back and remove the 1 from the data folder.
Install the apk and you are ready to go!

Comment any doubts you get, I'm glad to help.