Well it's welcome to the public sector again - and probably EU rules.
After all we the public are always complaining about non visibility of public sector stuff - and this is the result.
So the change requires it to be re-tendered by everyone.
In a private sector contract this would simply be a contract variation.
Most engineering contracts I've been involved with are signed before the details are finalised based on a rough tender outline anyway and it is accepted by both sides that there will be modifications en-route and price variations as a result.
Indeed this is done precisly because if all the details were fully finalised prior to the contract being awarded the two results would be
1. Delays in the contract and delivery time frames possibly impacting the financing of and viability of the project in the first place.
2. The costs of doing a fully detailed tender for which you might never get the contract would be huge resulting in large overheads from the wasted work and hence an overall increase in general contract prices to cover this expense.