I believe columns younger than 15 years are excluded from the PFI. Consequently I've seen a number of Residiums survive in West Sussex.It was meant to be the case that everything installed before 1990 got replaced, and columns installed after 1990 got retained. Of course, a number of columns installed after 1990 got replaced. A few older columns (excluding casties) have also slipped through the net, of which the oldest is believed to be a 1950s 25ft S&L column.
In Southampton and Hampshire, the intention was to replace or upgrade all lanterns, with functional lanterns replaced and modern or heritage lanterns either replaced or fitted with new control gear and remote monitoring. That hasn't quite happened yet, but not intentionally.
In West Sussex, on the other hand, the contract is different. West Sussex uses photocells rather than remote monitoring, and where columns are young enough to have been retained, it seems that quite often the lanterns fitted to these columns have also been retained, unlike in Southampton or Hampshire where the lanterns would have been replaced. Hence, West Sussex still retains a greater variety of modern lanterns, although I say variety in the loosest sense, as the vast majority of lighting is Libras, Arcs and Iridiums.