You can leave the seal on the front bearing and hope the internal sealed lubricant will last as long as you need it. Generally, the nose or front bearing does not have a seal, it may have a "shield." Shields are metal and allow lubricant to get into the bearing. Usually the front shield is retained 9to keep debris out of the bearing) and the rear shield is removed (to make access by the lubricant easirer). Seals are just that, seals, they do not allow for lubricant to get into the bearing and they rely on the seal keeping the bearing manufacturers lubricant to remain in place inside the bearing sealed area.

That is why main bearings are either open or shielded to allow for lubricant to get to the front bearing. Open or shielded bearings allow for flow though of lubricant. There have been cases where the main bearing was sealed and the front bearing was not, the front bearing did not last very long. And there is nothing wrong with using a sealed front bearing knowing the limitations involved. I would not use a sealed bearing for the main bearing.

It is best to replace bearings with like bearings. It is your engine and you can do what you want to do.

Cheers,

Chip