You could about it an entirely different way. I had to migrate to new filers from prod to DR as well, and it made more sense to work backwards (move DR first, then prod)
Break and delete the DR2 snapmirror relationship to DR1
Create a local snapmirror copy from DR2_agg1 to DR2_aggr2
once its complete, break the snapmirror relationship from DR_aggr1 to DR2 Aggr2
Expand DR2_Aggr2 to 10000GB
resync DR2_Aggr2 to DR1_Aggr1
Repeat for all the others. There would be some doubling up of resyncs as you move the middle volumes, but if the change rate isn't very high, it will be pretty quick to break, delete and resync once you get going, and you can have more control over the process than trying to do it all at once.