Our team recently migrated from Dynatrace appmon 7.0 to 7.10. We used Dynatrace migration tool for same.
Migration was rather smooth except one issue, the upgraded version was not storing purepaths. We were able to see purepaths during the tests but they would disappear as time passed by.
We traced the issue to be related to the storage space on C drive of Dynatrace server. C drive was almost full and it was holding lot of data from previous version of Dynatrace, when we removed previous version Dynatrace data from C drive, things were normal again.