If you have to do it, then I would also recommend Michael's code, but with an additional range check suitable to your needs, just to prevent valid but unreasonable dates.
e.g.
ALTER TABLE dbo.tablename ADD CONSTRAINT CHK_Date_format check
(case when SDate not like '[1-2][0-9][0-9][0-9]/[0-1][0-9]/[0-3][0-9]' then 0
when isdate(SDate) <> 1 then 0
else 1 end = 1 and SDate>='2000/01/01' and SDate<'2100/01/01')
However, while management may be right to have concerns about changing a data type, they should have those same concerns about adding a check constraint, which will throw errors for invalid data - something that presumably does not happen now...
MM
select geometry::STGeomFromWKB(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