I've always thought it impossible to fit a soldered joint into an existing system due to steam forming when blowlamping the joint. I recently noted my daughter had 'a proper man' in and he had done just that. What's the secret? I've always had to use compression fittings.
Turn water off and drain at lowest point, cut pipe, drain as much water out as possible if on a horizontal pipe try to bend pipes down slightly to drain fully If on a vertical pipe and the bottom pipe is full suck some water out with a thin tube before you start.
Clean pipe and fitting and flux the pipe only, assemble fitting make sure that when you are soldering If there is any water present that there is an open end higher than the joint to allow any steam to escape.
When you are working on dry pipework this is not a problem.
The biggest problem is that if you are working at the bottom of the system and water keeps dribbling down to your joint or if the stop tap has not completely turned the water off.
If there is more than the slightest water present it wont solder completely and the top of the fitting is soldered but the bottom is not and you cannot get enough heat into it to melt the solder again
In this case I would run all the new water pipe back to the new connection and just finish off with a compression fitting.