I would try pinning by origin, create /etc/apt/preferences with the following content:
Package: *
Pin: origin /repo.(pureos.net|puri.sm)/
Pin-priority: 1100
Then remove/comment out everythinng in /etc/apt/sources.list except for PureOS soruces, active content should looke like this:
deb Index of /pureos/ amber main
deb https://repo.pureos.net/pureos/ amber-security main
deb https://repo.pureos.net/pureos/ amber-updates main
After that do apt-get update and apt-get dist-upgrade. Pin-priority >1000 will cause apt to downgrade packages to PureOS versions.
The above is untested, so do let us know how it went, should you try it.
Once done, remove /etc/apt/preferences.