unmatch the syntax

i'm writing code that which syntax is very similar to vhdl however the
code is case sensitive. i would like to be able to made some
modifications to the syntax highlighting without modifying the actual
vhdl.vim. i understood that i need to put my syntax file into .vim/
after/syntax/vhdl.vim. however here is the issue:

original vhdl is not case sensitive but i need to make it case
sensitive, i.e

syn keyword vhdlStatement register

but i need it to be able to highlight only the lower case i.e
something like

syn case match
syn keyword vhdlStatement register
syn case ignore

the problem is that even i use the proposed solution my vhdl still
highlights Register because of the original vhdl definition. does
anybody know of a way to reset the definitions in syntax files?

any help is appreciated

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