One of our clusters is running 10.3.4.0 which currently can't be upgraded for various reasons.
NATS is manually configured by Skyline using the mesh calculator to connect each DMA to 4 other nodes each.
Newer versions of DM connects all node to all other nodes, is this possible to do the same with 10.3.4.0?
Is there anything else that would need to be taken into account if so?
Yes,
Even with 10.3.4.0 it should be possible to extend the configuration to connect all nodes to all other nodes.
The most important thing to keep in mind is that DM agents located in a geographically distant site (i.e., connected over a high-latency link) should not be added as such.

Thanks Pieter, that's what I was hoping, they're all in the same country with low latency links