Can I switch CSP partners for just some products and keep others with my current partner?

Technically, partner-of-record relationships are generally managed per product/subscription, so in principle you could move some products to a new partner while keeping others with your existing one. In practice, this adds administrative complexity (multiple invoices, multiple admin relationships) that most organizations prefer to avoid unless there's a specific reason for the split.

If you're considering this, it's worth discussing directly with both partners how overlapping access and support responsibilities would be handled.