Attachments question - help?

[Mr Grumpy]

They will still be in supply (assuming you are using default supply rules and not explicit supply) but will suffer the effects of being "detached" (out of the command range) of their parent HQ, you have effectively "broken" the chain of command to these units by using them outside the command range of their parent HQ and keeping them within the range of the next HQ up the chain (XXX Corps) will not fix this.