If vm is moved from standard swtich to dvswitch and both the switches are in the same network then is any downtime needed for the VM?
I don't tried but i think you can lost some ping, because you change of network card. I recommend to you ping with -t option on windows for test with one of your VM.
I've done this with over 200 VMs last week - sometimes I've lost one ping during migration, mostly there wasn't any network interruption at all. So, if everything is correctly configured (e.g. VLANs), there should be no problem doing this.
If all networking is setup correctly and the networks are the same you should be able to migrate the VM's from one switch to another. You may drop 1 ping during the migration but that's not enough to make TCP have a fit.