@MarcP I'll echo what @BrechtSchamp said, that non-Meraki transceivers are not officially supported, and add that if it's Cisco SFPs (the non MA-xxx product SKUs) running in MS switches, then it's a bit more involved, because the GLC SFP would be covered by the SMARTnet maintenance on a Catalyst switch for example, while the Meraki SFP would be covered by the license keys for the Meraki switches. They could still be RMA-able but it would be more of a manual process, so in the unlikely event you've got a GLC transceiver failure in an MS switch, open a Meraki Support ticket and reach out directly with your Cisco/Meraki account team to assist.
@Ritchie let us know if you still have any questions.