Cheating database (supposed) date by using nvarchar

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ASP.NET

SQL

DateTime

Hi.,
I have a field in my database that is supposed to be a datatype of datetime. but, because i can't get around that evil Conversion failed when converting date and/or time from character string while inserting datetime error. i have something like

[DateUP] NVARCHAR(25)

My search form now works using

"select * from tblUsers WHERE dateUP BETWEEN '" & txtDateUp.Text & "' AND  '" & txtDateUp.Text & "23:59:59'"

My Question are:
1. What can be the Negative implications that could happen?
2. Is this a total No-No? or it depends upon how you use it for? For me its just for logging and later on search and filter by DateUp.
3. If I use nvarchar for now. (If I still have time) Can I still change it to datetime without affecting the data inside?

What I have tried:

I only use this because I could not get pass the Conversion failed when converting date and/or time from character string while inserting datetime.
I have little time to finish this project.