I don't see anything in your question that indicates the original 50/50 split should be re-evaluated.
Yes, you can re-evaluate and possibly change equity splits in the future. However, depending on the original agreement, all owners would have to agree to the new allocations (something that may not necessarily be easy), and there must be a valid reason for doing so.
In the scenario you provide in your question, you indicate that you think the manager/business founder should get more equity than the developer founder. That doesn't make sense to me. Assuming they are both putting in equal (or just about equal) time in the startup, they should remain at 50/50. The actual roles don't matter, what's more important is the amount of time each founder puts towards the startup.
If you want the startup to succeed you need different roles. You can't have all developers, and you can't have all business folks. Does it make sense to give someone more equity just because of their skillset, when both skillsets are required to be successful? I don't think so.
Additionally, the risk is the same for both founders. Their roles in the startup don't make one founder hold more risk than the other. Presumably, both founders are spending the same amount of time in the startup, and have put in the same amount of startup capital into the business. If the risk is the same, it seems only fair that the equity be split 50/50.
I find it interesting that usually the business folks think their role is more important, while the developers think their role is more important. The reality is, both roles are important and necessary.
What, if anything, should happen to the split of shares?
In the scenario you outlined, nothing.
What would you do differently if you could go back in time to the moment the company was being formed? Would you formulate a different agreement?
If you are concerned about this you should look into vesting, but again the important factor is the time put towards the startup, not the roles. Vesting shouldn't change equity splits simply because of a change in roles.