Effect on Chain Geometry: Calculate rear wheel adjustment needed for all 3 setups (i.e. sprocket combinations)
Based on the needed sprocket distance, you can calculate how much you need to move your rear wheel either forwards or backwards.
In this case when you change your sprockets from Current (14-36) to Custom (15-38) setup, you will need to move your rear wheel 12,24 millimeters closer to your front sprocket. This is based on you keeping your current chain (or belt) of course.
The '<->' characters mean: move front and rear sprocket further apart, i.e. move
rear wheel backwards
This only works if you still have enough room to move your rear wheel in the
right direction. If not, if the rear wheel is already at maximum front or rear
position, the only option left is to get a new longer or shorter chain (or belt).