U won't have any choice but 2 drain the cooling system , as soon as u remove the hoses it will drain anyway
However , a tip 4 u , there is a drain tap on the bottom of the rad , place a container beneath this , open the tap (engine cold!) and then remove the rad cap. Most of the coolant will b drained this way so when the hoses r removed u won't have a gush of coolant over yr drive and best trainers
When u refill the system , run the engine with the rad cap remover 4 a few mins then replace a drive around 4 ten , stop and check/top up as required.
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The thermostat being refered to does not read the temperature. It controls the flow of water around the system. It is set so as to stop the water circulating until it reashes a set temperature, thus aiding quicker warming up of the engine. Fitting a high pressure rad cap changes changes the boiling point of the water therefore you need to change the stat accordingly.