If they had to untie the laces, and tie them up again, they would have a problem indeed! How tie your laces without being able to use your or the others' hands (which already wear gloves and are unable to help you with tying)? Unless a tying machine is used, which would be a waste of money and I'm not sure if technology was that far yet. And I don't think ...
There was no need for astronauts to adjust the laces. Laces were for sizing parts of the suit, not for putting them on or taking them off.
Unpressurized, the Mark IV was considered comfortable, and the plethora of straps and laces could be overcome by custom-fitting the suits to each astronaut.
Dressing for Altitude: U.S. Aviation Pressure Suits -— Wiley ...
For project Mercury, the astronauts wore the pressure suit for the entire flight.
There was therefore no need for astronauts to tie laces, as that can done by ground crew before launch, and then untied after splashdown.
Whether actual flown equipment used laces I know not. Some images do not have them, some do