Similarly to you I was moved to SL and GO early with some missing points, but I did write MPC an email on this matter as I was switching from star alliance to OW so I had no lounge access unless flying J.
On the SL email I had sent it was basically no issue at all.
For SL to GO I was missing 20 TP 6 months into the SL membership and I didn't want my next 90 TP long haul to be credited 20 TP and the remaining 70 going lost so I kindly asked them to move me up.
Now im 3 months into my GO membership and over half-way to DM but surely they wouldn't even budge if I wrote them a kind email.
You could always try asking them but unless you really need the DM membership immediately I don't think they would do it. Besides, it looks like you'll have no trouble getting there anyways...
I never asked for the tier upgrades from GR to SL to GO, Cathay just gave them to me quite generously with about 30% missing miles. My story is that I also switched from SA (top tier) to OW. But, I forgot to ask for a status match and just foolishly started from a mere CX Green Card with zero point. After getting GO rather quickly, I wrote to CX to request for DM based on my previous top tier status on SA for a very long time, CX did not budge either. Again, I am kinda of glad that CX is keeping the bar high so the first class lounge would have fewer people
