Hi,
I would use the main link - and leave the backup for the existing, willing to be decommissioned Branch/HQ when Migration comes to play, especially that should prove also the backups are working, so when your migration will move for the backup links to move, it i already proven.
Would do a phased and detailed migration Plane what corresponding with your current Technologies and Sections and the future ones, like:
"Existing config compare with the future Solution / one by one per each technology or protocol"
WAN
Cisco Routers - Meraki MX - Main Link move / adding 4G stick for check SD-WAN
DNS / Vlans / DHCP / ACL / Access restrictions / L3-L7 "policies" / Blacklisting / Whitelisting
then
VPN what allowed and what not
then moving towards down the Stack to Switches
Cisco Switches - Meraki MS
DNS/ Vlans/ ACL
then to AP`s / Cameras / to Endpoints.
All good ? - Backup link move / full decommission of existing
PPDIOO with Meraki , will cost more time on research for you first, but as every Engineer knows: measure twice , do once, stand back and optimize until its perfect.
🙂 Happiness !
kajcsu