Teamcity Upgrade throw error "Version mismatch: Configuration version is 544 but Database version is 454."

I am working on upgrading  our master server from TeamCity-6.5.6 to TeamCity-7.1.5).
Before applying change in live server i was trying upgrade/downgrade  in test server.
I was playing with upgrade/downgrade  between  teamcity version (TeamCity-6.5.6 , TeamCity-7.1.5)
I followed below step:
Backed up Database & Teamcity Data Directory of running teamcity version TeamCity-6.5.6.
Upgraded  to teamcity version TeamCity-7.1.5 .This requires upgarde of database during very first startup of webpage.Upgrade button was appear to perform database upgrade.
Backed up Database & Teamcity Data Directory of running teamcity version TeamCity-7.1.5.
Downgraded the TeamCity to version TeamCity-6.5.6.This required DROP of existing database and creation of empty database and restore of backup done in previous step.

All worked fine and all step gave expected result.then tried again:
Upgraded  to teamcity version TeamCity-7.1.5 .Now this  time i did not Upgarde button on webpage for first time .I got below error:

Version mismatch: Configuration version is 544 but Database version is 454.

I dont understand why this error as this is expected step database is in old version and that need to be upgraded in case of  latest TeamCity version.
The problem is resolved only by droping the database and creating the empty database. As i had backup before so restore the backup.

Any clue what went wrong why upgrade button not appeared after upgrading----->downgrading----->upgrading.
Deleting the database is scary step.Please help.

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