I suspect that MS have recently marked the 6150 as incompatible (and the time you performed the original installation, they had not done so).
The compatibility check seems to be programmed to say everything is OK UNLESS it finds something (hardware or software) which it has positively identified as incompatible. This would be in line with Microsoft's "bully boy" tactics in (1) pushing Win 10 at every opportunity, (2) making it difficult to avoid the upgrade pressure, (3) getting the users to do the testing.
It would have been more appropriate, really, to have said something like "we have identified that your computer contains the following hardware/software (x) which is incompatible with Windows 10, and the following hardware/software (y) for which we have not yet determined compatibility with windows 10 - visit the manufactures support website to determine compatibility of these items". As user testing/telemetry (point 3 above) identified that 6150 owners were having issues, the 6150 would have moved from list "y" to list "x". Instead they let users fall into the trap that you did, determine that the 6150 was incompatible, and then updated their compatibility database to stop other users having the issue.
You may have the option of purchasing a display adapter that is windows 10 compatible, however, given that your system is obviously aged, You may want to think carefully about whether it is worth doing that, certainly once you did that and performed the Win 10 upgrade, you may find that any further upgrade of your system altered the hardware fingerprint of your system (that MS tied your Win 10 upgrade to) in a way that Win 10 would no longer activate (and you'd need to buy a new copy of Win 10 or go back to your earlier OS (if indeed that could be made to activate after the changes).