They definitely won't shut down their entire HSPA network. I'm just talking about the 1700 MHz spectrum. Currently, with T-Mobile, that 1700MHz band is used for their HSPA+ network. Once the buyout goes through, AT&T will use that 1700MHz band exclusively for LTE (keeping HSPA on their 850MHz band I believe). None of the current T-Mobile phones support LTE or HSPA on 850/1800MHz, so they wont work once the 1700MHz LTE rollout begins.
I know both T-Mobile and AT&T share some bandwidth in the 2100MHz spectrum, but I'm not sure how popular this spectrum is or even if it's used for data. There's a slight chance that some T-Mobile phones will still be able to use the 2100MHz band for HSPA, but I'm pretty sure the 2100MHz coverage is pretty minimal in the US.