select cast(replace(cast(xml as nvarchar(max)),'old@mail.be', 'new@mail.be') as ntext)
from channels
where xml like '%old@mail.be%'
/*
update channels
set xml = cast(replace(cast(xml as nvarchar(max)),'old@mail.be', 'new@mail.be') as ntext)
where xml like '%old@mail.be%'
*/