And if you want to keep one dimension the same, but leave the other as it is, just leave the persistent dimension out of what you typed.
So for example if you use comma as a separator, and have set a dimension of (say) 600,300 and now want to change it to 600, 400, just type ,400 before you change to another tool. Don’t click in the Measurement box first, just type.
Or if you wanted to keep the 300 dimension, and change the 600 to (say) 500 just type 500,.
If you are using the semicolon as a separator, type ; instead of ,.
And if you are using measurements with a decimal point, in regions where the decimal separator is full stop (or period, in US English), and the delimiter is comma, just type it as normal - e.g. 620.5, 300.2, or where the decimal separator is a comma, and the delimiter is semicolon then type 620,5; 300,2.
Be aware that in all but the latest one or two versions of SU, using Imperial/US Customary units when the model units are metric will give you inconsistent and often WRONG conversions.
Whoever first programmed that was having a seriously ‘off’ day.