Was a pain in the b.....
Not sure if this tool still works I haven't used it for quite a while (about 325 rides ago

):
MapMyRide GPX Converter
You have to convert each MMR ride 1 by 1 and then import into Strava 1 by 1.
"Luckily" when I decided to change I only had 100 rides to transpose across and just did over a period month by month.
This tool has been up and down from what I can see on the web, hopefully it's up at the moment !
Hope this helps.