I'm not sure what your HR would think was a basic courtesy - a lot of companies just see 'secretary' as 'admin staff' who can just be swapped around at will and if there are no changes to the terms of your employment, they may not see any need to formalise the move in writing.  This is probably to your benefit. 

I wouldn't chase getting something in writing unless I knew that I was completely happy with the new arrangement.  It might trigger a new contract and you have 30 years service* which is significant if there are further reorganisations or you decide that the new role isn't for you.  Is there a trial period?  Perhaps the end of the trial period is when you could ask for written confirmation of the new role or perhaps at an annual review if you have one?   The person who notified you of the change of role is probably who you could talk to in the first instance, but do make sure you're happy, and DON'T trust HR to do the right thing for you - they will do the right thing for the company. 

*actually on rereading your post, I'm not sure if your 30 years service is with the same co, but the principle applies, protect your service term and your original contract if the terms are favourable