If I rendered in cycles with 128 samples for example, is there any way to increase the samples count to 256 and only do the additional 128 samples without starting the rendering all over again? I'm using version 2.83.
You can enable Progressive Refine (Render Properties > Performance > Tiles), set the sample count to a high amount like
10000 and stop the process manually (ESC) when satisfied.
However, I'd suggest use the Render Region feature and drag a rectangle around the darkest spot in the image (CtrlB), set the viewport samples to
0 and wait until the noise is resolved to get an idea how many samples are required and do a full render based on that knowledge.