Entity Framework 7 RC1 Reverse Engineer Entire DB only?

are there any options for reverse engineering just some of the tables in the DB?

Yes. To see options, execute get-help Scaffold-DbContext in the Package Manager Console. This is the output for 7.0.0-rc1-final. To limit selection to only some tables, use the -Tables options

PM> get-help Scaffold-DbContext

NAME
    Scaffold-DbContext

SYNOPSIS
    Scaffolds a DbContext and entity type classes for a specified database.


SYNTAX
    Scaffold-DbContext [-Connection] <String> [-Provider] <String> [-OutputDirectory <String>] [-ContextClassName <String>] [-Schemas <String[]>] [-Tables <String[]>] [-DataAnnotations] 
    [-Project <String>] [-StartupProject <String>] [-Environment <String>] [<CommonParameters>]


DESCRIPTION
    Scaffolds a DbContext and entity type classes for a specified database.


RELATED LINKS
    about_EntityFramework 

REMARKS
    To see the examples, type: "get-help Scaffold-DbContext -examples".
    For more information, type: "get-help Scaffold-DbContext -detailed".
    For technical information, type: "get-help Scaffold-DbContext -full".
    For online help, type: "get-help Scaffold-DbContext -online"

if we change the DB and want to reverse engineer it a second time what is the correct procedure?

Delete your old model classes and context and re-run reverse engineering from the updated database. Scaffold-DbContext does not make in-place updates in RC1.