How close can soldered and threaded joints be?
If I cannot unscrew the pipe and solder it I will wrap a very wet rag to the end that has the combination plastic/copper fittings and sweat the new fittings on. I use Mapp Gas as it is hotter and works better with non lead solders than propane. I do burnish the pipe and fitting with abrasive and use water based flux. I have done this as close as 3" to a composite fitting without damage. If you notice the rag starting to steam you may need more water on it “dripping wet” keeps the heat from traveling and melting the plastic parts.