1.4.1 agent to 1.5.x consumer event/message compatibility?

We have a lot of 1.4.1 agents deployed, they basically consume file events (lightly annotate them with a few fields) and then forward as-is onto Redis, and then are consumed by another 1.4.1 logstash instance that does heavier manipulations.

We are looking to upgrade the latter logstash instances that consume the messages generated by the agents.

My question is, are there any fundamental message/event structure format changes between 1.4 and 1.5 that would prevent us from doing this?

No, there are no major differences. The only difference I can think of is the introduction of the @metadata field but that field is ignored by outputs so you probably won't even notice it.