This is a good resource for info on 5.1 mixes: http://duc.avid.com/showthread.php?t=87830
Often a stereo sound mix for TV uses limiters and compressors and has less dynamics than a theatre mix. This is because it is mixed at a 79 dBC per channel instead of 85 dBC per channel.
A movie theatre mix is also mixed with 20 dB of headroom.
A TV mix converted to a 5.1 movie theatre mix will have lower dynamics. The best option is to go back to the stems/sources that were used to make the TV mix and mix it again at 85 dBC per channel.
I recommend having all movie theatre mixes done at a professional film mixer that uses the 85 dBC per channel standard.
If you are converting a stereo TV mix to 5.1 mixes for DCP use I recommend using Pro tools and an upmixing plugin. You can match the dialogue normalization level, but not the dynamics of a proper 85 dbC movie theatre mix.