I think the most important signal will be WHICH uniform they turn up in. Andrew famously wanted to wear his honorary admiral's uniform to the funeral of the Duke of Edinburgh, and got a flat no.
I suspect that the uniforms they will be wearing will be the far more modest ones that they actually wore on duty right before retirement. For Andrew, that would be the dress uniform of a Royal Navy Commander, and for Harry, the uniform of a Pilot Caption in the Army Air Corps. If so, it may be that Charles didn't actually make the decision. As retirees honourably discharged, they may have been given special permission directly by those services to wear them for this one occasion, if it was applied for as part of the Queen's funeral plan request. It's definitely not the norm in the UK services, but if the Queen personally requested permission to let her retired son and grandson wear their service uniforms to her funeral, I think that the Army & Navy might have been willing to grant an exception. This is what those service uniforms look like.