I have a lot of identical parts in an assembly. On these parts, I have to add (weld) other parts. They will therefore become an assembly. I would like to replace all the sparts present in the main assembly, the s^parts with the new assembly.
If I understood the question correctly, when you have your welded parts assembly (having formed a subassembly in the assembly for example), you right-click on one of the parts to be replaced, "Replace components" and you select your welded assembly.
You can also check the "All occurrences" option.
On the other hand, your constraints may be lifted...