While clothing does not define a person, people throughout society use clothing as a manner of expressing their identities. Making a dramatic change in dress is usually associated with the individual making a dramatic change in personality or role in life.
This is even more true in the Orthodox community, where people are routinely identified and even referred to by the clothing they wear. A Litvish man is often called "black hat" and it is not uncommon to hear statements such as "Sara is marrying a streimel", when she is actually getting married to its wearer. For this reason, when an Orthodox person changes his or her way of dress it makes an even bigger statement than it does in the secular world.
Therefore, when a person does not feel comfortable identifying as non-religious he or she makes a point of keeping to the same standard of dress as before. Conversely, if a person wants to show that he or she is more modern and/or not frum altogether, the message is often intentionally broadcast through clothing choices.
In my case, as I imagine in many others, this even applies in private. I still often dress tznius at home even as I am committing aveiros. The only explanation that I have for this is that I am simply not comfortable (yet) with identifying as non-frum. I still identify as a member of the frum community despite not wanting to be part of it.
This probably also explains why some people who have recently gone off-the-derech dress in a manner that is more provocotive than that of general society. By dressing in a manner that is the direct opposite of tznius, it expresses that one wants to be sure that she is not identified with a community that she despises. Even among those who are still frum, those who simply wish to be more modern typically change their dress more than anything else, such as kashrus standards.
So, readers, what do you think? At which stage of your departure from obervance did you change your manner of dress partially or totally? Were you keeping shabbos while dressed in a non-frum (by anyone's standard) manner, or did you break shabbos while dressed as before?