A 12 speed cassette will fit your current old 26er inch wheel fine as long as it starts with an 11 tooth cog. What won't fit your current HG freehub body are the cassettes that start with a 10 or smaller tooth cog. Shimano and Sram made their own fancy freehub bodies to fit those 10 tooth cog cassettes but sunrance still makes normal HG freehub based casssettes all the way up to 12 speed.
https://www.jensonusa.com/Sunrace-MZ...SPEED-Cassette
like that for example