Does DVDtoBD change Video resolution? Will destination Blu-ray disc have different quality from original DVD?

No, DVDtoBD does not change quality or resolution of video. But you can manually replace Video asset with new high quality asset. You do not need to change another part of BD project to do this. From another hand, DVDtoBD will change Aspect or some another fields of Video file for compliance with BD specification.

How will be look original Menu and Navigation commands in destination BD project?

It will look as it was in original DVD. DVDtoBD re-code Menu and all Navigation commands to Blu-ray format (Interactive Graphic). You will receive the same Menus but with BD functionality.

Will DVDtoBD work if original DVD was not created using Scenarist SD ®?

Yes, it is does not matter what was authoring platform for original DVD. Moreover, DVDtoBD will enable some corrections if it is necessary.

What will be with Titles/Audio/Subtitles menus in destination BD?

Blu-ray disc can not have separated Titles/Audio/Subtitles menus as it was in DVD. But you will have access to these menus through Main menu.