GDPR is not in any way ambiguous there, take a look for yourself [1]. Keeping an eye on those changes is a part of your responsibilities as a data controller, it's your vendors' responsibility to inform you of any changes, and it's your responsibility to vet vendors for GDPR compliance. Again, if your lawyers didn't explain this to you (and you haven't read the law yourself), I'd be very cautious of those lawyers.
On the other hand they probably realise there's zero chance for substantial review of your GDPR practices by the regulator (much less seeing them in court), so they can recommend sticking a useless plaster (opt-in has to be specific, and how can it be specific if you collect it for unknown future changes) and keep you in the dark about more substantial requirements.
GDPR is a very good and clearly stated law, you can read through it yourself in about half an hour to an hour, a negligible time investment for such an important piece of legislation. The purported ambiguity is a psyop by people who don't want to comply.