It sounds like this client knows what they're doing. VFP 5 is a very fine product.

>Building indexes in VFP6 is three times faster than VFP 5(they have some tables around 10 million records)
Typically, an index (tag) does not need rebuilding unless something is broke.
>
>Scroll bars on the side of forms can makes things easier for end users
Sometimes, scroll bars on the side of forms can make it more difficult for end users.

>VFP 6 has a built in framework that can be used for some things
Find that "some thing(s)" and propose it.

>In this kind of situation, are there any other solid reasons for switching to VFP 6 that can improve things RIGHT NOW, not "if you expand in the future".
In this clients case, probably not...
>
>TIA