Ruscitto,
You question is somewhat unclear, but you appear to be on the right track.
Per API 1104, Appendix B, 20th edition, a welder who has an existing multiple qualification to 6.3 and an in-service qualification to B.3 should be qualified as an in-service welder for all positions, all diameters, and all wall thicknesses within the limits of the essential variables in 6.3.
Be aware that this requirement changes a bit when PHMSA adopts the 21st edition of API 1104 as the requirements become more stringent. For now, though, and until PHMSA does adopt the 21st edition of AI 1104, your reasoning holds.