Does the central bank ask the government to pay or are they rolled over?
Opinion poll from focus economist asks similar question about Fed and ECB QE. Some respondents say QE will be phased out slowly others just say they will continue holding those assets for long time.
Either of these two options can happen in principle. In fact there is even a third option where central bank just decides to retire the bond - meaning government never has to pay back or roll over (in some countries there might be legal constraints on this option but I am talking in general terms).
What will any central bank decide to do when that time comes will depend on the discretion of whoever is at particular time appointed to lead the central bank.
The general idea is that the government pay the principal on the bond to the central bank and that money then disappears out of existence (i.e. the opposite of the money creation that happens at the beginning of the QE process). But in practice, this process is so painful that governments everywhere have simply been issuing more bonds and then doing ever more QE to compensate. This process has been going on for decades in Japan.