Promotion in a Vexilatio

[Nathan Ross]

not so many soldiers or normal centurions were deployed from the legions to the praetorians before Septimus Severus.

There were quite a few of them from the 1st C onwards, as shown in career inscriptions - the transferred centurion served three terms in the Urbani, Vigiles and Praetorians respectively, and commonly (but not invariably) moved straight on to the primipilate. The earliest known case, I believe, dates from the Neronian era but the practice really picked up in the 2nd century. Obviously it was limited by the number of Guard centurionates available, but it seems to have been a way of fast-tracking capable (or well connected) centurions into tribunates and higher equestrian posts. Dobson, Mann and Summerly discuss this, amongst others.

But, as you say, this isn't directly relevant to the appointment of centurions 'in the field'...