Here's a pretty straightforward video on how to do the following for a TWSBI nib:
- Swap out the whole nib unit/grip section for different nib sizes
- Swap out the nib unit within the grip section for pens that have colored grip sections
- Replace inner cap liner for newer TWSBI nibs (especially stubs) that are slightly wider than the original nibs on older (pre-February 2012ish 540's and all 530's)
- Use the black body cap (comes with each separate nib unit purchased) to cover inked up TWSBI pen bodies during nib swaps
Hopefully this is helpful to you! The process is very simple, and you'll have a much clearer understanding of how it's done once you see the video. No special tools are required. Post any questions/comments below!