Migration to a new core issue

hello guys

I am trying to Migrate old cores to EX9200 core as mentioned in the picture below

i connected the old cores with the new one through trunk to expand the network and start Migrating the servers to EX4300 switch with minimum downtime.

the issue im facing now is i can not ping the VRRP ip on the old core from my PC2 but i can from PC1

I assume its arp issue how can i solve that.

and whats the best idea to migrate smoothly with minimum downtime for all servers

note the WAN is fiber link to another site connected via OSPF.

thanks in advance.