I already have a directory with the entire source code for LineageOS, which was initialized with:

repo init -u https://github.com/LineageOS/android.git -b lineage-15.1

After a couple builds, the directory is over 70GiB.

Now I would like to build Android-x86 (the LineageOS branch/port), and I'd like to know if there's a way to "rebase" the existing source code instead of re-downloading it all over again.

The Android-x86 code base is initialized with

repo init -u git://git.osdn.net/gitroot/android-x86/manifest -b oreo-x86

I tried running this from the same directory but got as a result:

fatal: manifest 'default.xml' not available

fatal: remote github not defined in /media/Fast/Dev/android/lineage/.repo/manifests/default.xml