Did you install fresh NetBSD boot blocks after upgrading the partition 
ID? That is critical. see installboot(8).

"Eric Delcamp" <e.delcamp@wanadoo.fr> writes:

> Hi !
> 
> About bootblocks, I have the old BSD-ID (165) on a partition. If I change it
> to 169, I can't boot (no bootable partition).
> The problem is that I have Win95, BeOS and WinNT on this disk, with the NT
> boot-loader. I don't want to erase all bootblocks, just want to put a new
> bootloader for NetBSD.
> How to do this ?
> 
> Thanks.