MSSQL/ASP migration

I have a customer who is wants to migrate his MSSQL database to
PostgreSQL and we'll replace his application ASP with PHP. The issues
should be limited as there are no stored procedures or triggers in
MSSQL, just structure and data should be all that is needed to migrate.
I have never migrated from MSSQL or to PostgreSQL, but have handled
database migration in the past for other DB's. I know there is
mssql2pgsql script out there somewhere and I find lots of info on the
subject. Of course, when we rebuild his application, some db structure
will change, I was planning to do all the changes to structure after a
successful migration of his current structure now w/o data. After the
new application is done, then create a migration path for the data. Is
that the best way to handle these types of migrations? The customer will
want to continue working on the old system during the several months of
development in the new. Any docs or other helpful info is welcome, just
looking for some advise.

Thanks in advance,
--
Robert