Hello Aspirant! 

You can opt for B.Tech. in Civil Engineering after pursuing Diploma in Architecture. But as far as M.Tech. in Civil Engineering or PG Diploma in Civil Engineering is considered, it is dependent on the eligibilty criteria of the concerned institution. Usually, institutions do not take admission into M.Tech or PG Diploma in Civil Engineering if candidate has done Diploma in Architecture.

Hey,

Yes. You can very well do it. You just need to give JEE mains paper 2  to get into several government or private colleges. But how hard you may try, you will not be making it into NITs because  the diploma candidates are denied entry into NITs or other government institutes except for IITs.

 It's not that hard though. But remember, Architecture is all about the outer beauty. Civil Engineering is all about inner strength.

Hope this helps. 

Good luck.