[support] Folding old site into Drupal structure

[Tenant]

We run our own server (Debian) with many virtual domains. For many of 
the newer sites using Drupal, we've followed the protocol of 
installing Drupal -- in /home/drupal/public_html -- with each site 
getting it's own directory in  /home/drupal/public_html/sites/mysite.com.

For all modules and themes we install in 
/home/drupal/public_html/sites/all/modules (or themes) and that works 
very well.

Works very well as long as we point mysite.com to 
/home/drupal/public_html  in Apache vhosts.conf. All this certainly 
takes a load-off from reinstalling updates in each domain.

However we have a few domains that are quite old (mid-90's) and have 
extensive hierarchy of static files. We would like to upgrade those 
sites to Drupal over time. So here's the question...

I could overlay Drupal in /home/old-site/public_html, then use Front 
Page to keep the old site look & feel and even use menus to point to 
older pages and use this setup while -- over time -- we would convert 
old material to Drupal pages. And vhosts.conf would stay as it is for 
that domain.

But doing so would require doing twice the number of 
core/module/theme upgrades, a pain I'd rather avoid.

Alternatively, could I  install all the old site content files under 
/home/drupal/public_html/sites/mysite.com - and then change vhosts 
and get the benefit of single upgrades?

Or (third option) could I use symbolic links under  /sites/mysite.com 
that link to the old site files and directories?

Are there other ways to approach this dilemma? Thanks for any thoughts on this.

Jeremy