When you change the area (from 0 to 3) on one of the interfaces, the distribution router is most likely to prefer the interface in the area 0 for the prefixes learned over both interfaces since OSPF protocol dictates that intra-area route be preferred over inter-area route (for the same route).
You may have to wait for few seconds after changing the area on the core side interface (assuming the network management station is behind the core routers) to regain the connectivity. This is due to the time required for the OSPF to converge.
Manish
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