I might be cynical but having worked in a music store and having had dealings with Andertons, this has “here’s a stock answer that tells you not to do it so it saves us any backlash when you try it and it goes wrong” written all over it.
Andertons are generally pretty clueless about bass so I wouldn’t take their advice as the basis for your decision (especially when the manufacturer has confirmed the opposite/truth!)
IME, I bought a Sire P7 V earlier this year from Andertons and the pickup covers come off and could be replaced (although I haven’t taken the time to see if they are ‘standard’ size)
Also, it might be helpful to know that whilst the Sire P7 V split coil pickup is ‘shorter’ than a standard replacement set (like Aguilar etc), the routing in the bass is larger to accommodate any upgrades, you’d just have to get the pick guard hole made larger to match the pickup.