Use Replace() In An Update In A Large Ntext

I've got a table that I have to update in preparation for our environment move (2k to 2005 SP2). The developers that designed the application created a table called schemas, which holds the contents of an XML file inside of an ntext field named Data.

I need to parse through the field and do a find/replace to replace all instances of www.site.com with www7.site.com. It's all over the place in the file. The problem is, that the datalength() of each of the fields (there are 2 rows) are above 15000.

normally, I'd run something like this:

update schemas
set data=replace (cast(Data as varchar(max)),'www.site.com','www7.site.com')
where data like '%www.site.com%'


Smaller columns it works great - but it won't work on these because they're too big (the update will chop anything beyond the varchar(max) value). I could do it manually, but this DB will be refreshed from production on a weekly basis and I'd like to script as many of the environment changes to the DB as much as possible.

Any ideas?