I have an old (~20+year) Mont Icicle downie that could do with a bit of a refresh

I have had a look around both here and on the broader interwebs for any guidance but not found anything in particular that would help with my question which is..
Has anyone got any guidance or advice on a reasonable way to add fill to an existing jacket? Looking at the stitching pattern this might have to be done by cutting a slit, adding fill, sewing / gluing shut.
Any advice, experience, etc anyone could share?
I've never worked with down before, so any pointers on working with down generally also gratefully received

TIA