The ammo makers have already increased production. There are many sources for 155mm shells. Nammo [1] BAE Systems.[2] Dezamet [3] General Dynamics [4]. Those are somewhat generic. The real shortages are in the fancier guided stuff. Most of which is decades-old technology and could be built today from common smartphone parts.

[1] https://www.nammo.com/product/our-products/ammunition/large-...

[2] https://www.baesystems.com/en/product/shell-155mm-how-he-l15

[3] https://dezamet.com.pl/155mm,26,en.html

[4] https://www.gd-ots.com/munitions/artillery/