AAA-ZZZ gives 17,576 combinations (based google search), there are 16,465 on the current register which includes alphanumeric regos.
The 17,576 combinations doesn't include the removal of in-appropriate regos and reserved rego blocks like VH-ZNx series for QF 787s.
When QQ started to register their E190's they had ask rego owners in the block they wanted to use to change regos. I expect this was at QQ's cost.
For those who are old enough to remember Y2K, this caused an enormous COST as each program had to scanned and checked for the occurrence of an YY instead of a YYYY year field. Once found, programs had to changed, tested and re-integrated.
To change VH-XXX to VH-XXXX is a MASSIVE undertaking in IT COST, similar to Y2K.
Also, there is a hardware storage cost (disk space) which needs to be considered.
I can't give you a $Aus value as I've I've been out of the IT industry for a while and haven't keep up with the current pricing.
To go to an alpha character, is a work-around to save costs.
CASA updates the available marks on a regular basis.
https://services.casa.gov.au/CSV/availablemarks.pdf
Will we go to VH-XXXX ? I don't know, but it will COST.
It is not as easy as it sounds.