JCT: What happens if a Provisional Sum is too low?

A provisional sum has been included for finishes. The actual requirements are being finalised by the Client and it looks like the quality will be much more than expected and the value approximately 10X the original provisional sum. The expected new value for finishes alone is likely to be so high that it will equal the original contract sum. (effectively doubling the contract sum)

Due to the size of the change can this reasonably be retained within the original contract? Does the Contractor have reasonable grounds for pricing additional prelims to cover the additional resources required for this work on top of the %age mark up for O&P?

Assuming JCT SBC16

Provisional sums are priced in the same way as variations (5.2), so if the amount if ‘too low’ it is only a concern for the Employer’s budget, the Contractor will be paid as if it is additional work.

I assume the provisional sum is ‘undefined’ (as it is now considered to be an incorrect assessment of the value), then the Contractor is also entitled to an extension of time ( and loss and expense (