Upgradation & Performance Guarantee Issue

Dears
We are in negotiation of being involved in an upgrade of an existing plant.

The upgrade was necessaciated when the Employer finds himself that the plant is not meeting the "Performance standards". The existing plant was commissioned some 3-4 years back.

Now, the employer wants us to present him a Performance Guarantee of the whole plant compare to our offer of PG of the upgrade only.

This will not obviously come free of cost. But, what other reasons I may come up with to avoid an over burden of such a situation.

Please note that the contract terms would be FIDIC Yellow.

Thanks in advance

Regards