Moving from a small package to a bigger package will always be possible, as assigned pads will always be present in the bigger package.
On the contrary, moving from a big package to a smaller one may reveal some errors, as there are less accessible pads in the old package than in the new one.
In such cases, the user will have to fix allocation errors inside the textual configuration, removing configuration that are not possible, or assign functions to other, not allocated pads.
The swap in itself is done using the textual editor, changing the package name (for example C4 in the example below) inside the board file.
You can choose the package you want to use from the list of available packages, using the CTRL-SPACE (or CTRL-TAB) keyboard combinations as usual.