excel is updating links when told "dont update"

we are using excel worksheets for calculating costings for construction work.

the worksheets contain links to other excel worksheets with prices etc.

if a project goes quiet for a few months and then needs re-pricing, we
re-open the worksheet and save it with a different name, then close it and
"update the links" this updates the prices to todays rates.

however if we open the original file to compare final figures and click
"dont update" then excel still changes a number of links and appears to read
them from the newer file. this then messes up the original costing. the
obvious answer is not to have them open at the same time, but can anyone say
why, when you click dont update does excel read some of the references and
update them (its usually with a specific group) therefore messing your
original worksheet up.