>> ... stating that HP has informed them that HP dropped support for forms in late releases of firmware ...
Assuming that by "forms", this is referring to the use of PCL5 macros, I find this hard to believe, although, as I have no access to the printer models you refer to, I have no way of testing this.
The README file (see http://ftp.hp.com/pub/softlib/software12/COL41452/sj-108451-7/readme_ljM4345mfpfw_48_306_1.pdf ) for the LJ M4345 firmware upgrades does not (as far as I can see, from a quick perusal) make any mention of changes in how macros are handled.