Yes, due to a complete merge of three different products, it is not possible to upgrade from standalone BV (Monitor or Reporter) to Veeam ONE.
1. Yes, you can use vCenter Server custom attributes for that. If you have these attributes applied to your VMs, then the only thing you should do is to map these attributes from vCenter Server to Veeam ONE v6 database.
2. It's better to use a clean machine for that, as Veeam ONE is using the same ports as BV, besides I'm not quite sure about collection services coexistence.