A shared foundation
Crawlability, clean structure, accurate entity data and genuinely useful content serve both conventional search and generative answers. Most of what is sold as GEO is work that a competent SEO programme was already meant to cover.
Where they diverge
Conventional search still rewards breadth: covering a topic set, earning links, holding positions over time. Generative surfaces reward precision and citability, and they are far less forgiving of claims that cannot be traced to a source.
They also fail differently. A weak SEO position is invisible. A weak GEO position is worse, because the assistant will still answer the question, using somebody else.
Budgeting for both
Fund the shared foundation once and properly. Then split the remaining effort according to how your buyers actually research, which is a question worth answering with data rather than assumption.