Have you come across Shoben tape yet?
This stripey black and white tape sticks well to fabric and is very narrow - I use it to design new necklines or bust seams on toiles or garments I want to alter - like this:
(this is Ralph Pink on Youtube designing seam on a bustier with Shoben tape)
Then you can mark these changes back to your pattern using a sharp spikey tracing wheel (and board underneath - a cutting mat or even just a piece of thick cardboard box will do).
You can buy Shoben tape online for £7.50 from Sew Essential (it will last you years unless you're making numerous corsets like Ralph above!) here
Buy your own tracing wheel online here