We have done quite a few of these upgrade/ recommissioning jobs now on undersized/ badly specified package sewage treatment plants, and they are all still working well. As far as I know, we are the only ones offering this service.
We've also had an expert witness enquiry to do with package plant problems, as is so often the case specified and installed by monkeys who went bust not long after taking the client's money.
If you want to save yourself money, involve an expert at specification stage. Salesmen for package plant companies are not generally speaking any more professional engineers than the guy on the yard at Jewsons. You should not assume that their opinion will count as professional advice in court, and you will in any case find that they aren't actually guaranteeing anything. Neither should you assume that they will still be in business to deliver on any promises they might make.