Family Historian User Group

After importing my TMG database to FH 6.2 and later upgrade this version to 7.09, I discovered that a lot of my data on people and their connection to the parents is marked as an A (adopted) instead of B (birth)

Is it possible to search and change this to the correct connection, on a group of people.

Yes, use the Search and Replace plugin from the Plugin Store.

Run the plugin and set the Search Scope to Individual Records (INDI) then use Select Records to choose the child records you wish to change from Adopted to Birth.

On the Extra Filters tab, click Exclude All Short Text and Exclude All Long Text.
Then tick just All other one line Short Text fields lower left which will include the Pedigree (PEDI) relationship data.

Return to the Major Options tab and in Search: enter Adopted and in Replace: enter Birth

To check the correct items are being found, use the Search ONLY button and use Report to produce a list of all matching items or Skip to just eyeball the items. Untick Confirm every item found and/or use Cancel to complete the search.
The plugin will remember your settings between runs.

When happy, use Search & Replace to perform the replacements with similar options to the search.
The final Result Set will list every changed Pedigree (PEDI) item.

If anything is unsatisfactory use Edit > Undo Plugin Updates before closing FH to reverse all the changes.

BTW:
If the Pedigree (PEDI) relationship is Birth, it is more common to leave the item blank (the default).
To achieve that setting leave the Replace: field blank in the above process and the Adopted entries will get removed.
However, there is no going back from that status once FH has been saved and the settings have become established.
In the short term you could return to the Adopted status using a Backup or a Snapshot but all other changes would be lost too.