What has usually happened, in my experience, is that the recruitment freeze causes shortages in key positions in mid and higher level grades. Eventually, some/many of these are plugged by temporary acting up which leads, often, to permanent upgrading by length of time in the higher grade, usually 2 years. This, in turn, leads to further shortages at ground level and the freeze will be eased to fill these and remaining higher level vacancies. So, if you hang on, you could advance relatively rapidly.