There is not currently an upgrade path available from BTS to PWDM, however, you can use both versions side by side on the same project during the transition.

https://communities.bentley.com/products/projectwise/projectwise-deliverables-management/b/projectwise-deliverables-management---blog/posts/did-you-know-you-can-run-bentley-transmittal-services-and-projectwise-deliverables-management-on-the-same-project