So there is precedent. Most interesting.The ADF has always reused equipment, the Bofors on the Fremantles were rebuilt 1940s guns on a new ADI mount, the torpedo tubes on most of the ANZACs were from the River class DEs / frigates, with the exception of the new units ordered for the AWDs the RANs Phalanx are part of a pool, that is being refurbished, including those from the FFGs and LPAs (Kanimbla and Manoora were each delivered with one fitted). It makes sense when you think about it and is also quite common overseas, i.e. the Mk-13 GMLS on the French Jean Bart DDGs were refurbished units from 1950s vintage destroyers that were converted to DDGs in the 1960s. So realistically there is no reason why we couldn't do the same with existing systems, in particular the eight cell Mk-41 VLS of which there are twelve.
Actually on the French upgrade of older gun destroyers, one of the reasons Tartar was the RANs prefered system as well as why there were only three DDGs and that they were built overseas (a fourth was very seriously considered) was that we planned to convert both the Daring and Battle class destroyers to DDGs. Competing systems such as Sea Slug and Terrier were to large to retrofit to such small ships and it was assumed our yards would be too busy with conversion work to build new ships. These were not the only reasons but they were up there, as a force of eight or nine DDGs was seen as vital at the time.
CoA's appetite for technical risk involved in re-use of equipment and systems will be key though. Unless its provided as GFE and the integration risk and performance warranties then passed onto the prime. That could also work.