The Theravadin tradition is a very conservative tradition. There is a reason for this in that many times people change things due to their defilements, not through any particular valid reason. As far as the rules of the monks, they have changed in the sense that there are different vehicles, etc. and the necessity to go further than walking can allow. The Buddha gave some leeway for this by saying to one monk before he died that the lesser rules don't have to be followed. I'm not sure of the particular words he used in the scriptures. However, the more important rules do not change because basically they wanted to keep closely to the Buddha's words, and not assume that they know better about what purifies the mind.
We have to be careful about changing rules of morality with changing times. Basic rules of morality, the Five Precepts, are as helpful now as they were two and a half thousand years ago. There has been a tendency for some people to want to change these rules to satisfy their own particular cravings. I read in one book about "refining" the precepts. But their refinement of the 5th precept was that people could use alcohol and take drugs as long as they weren't teaching a retreat. I don't consider that a refinement. I consider it a corruption.
So we have to be careful about making the Dhamma acceptable for the masses. It's not to say that I don't agree with democracy, but if we want to change it and corrupt it to a lower level just to make it acceptable, then it doesn't provide a guideline for those who want to reach for a higher level of practice. When we look at the Buddha's teaching it has these different levels already in it. He taught people from all walks of life and had different levels for all these different walks of life, different precepts.
If we want to purify our mind we have to be careful, particularly with the 5th precept. Although this precept is not always acceptable to some Westerners, if we value our awareness we know it's difficult enough to practice when our mind is clear, let alone to actually cloud it with something else. So to actually say that teachers, who are supposed to be an example, can change and lower that precept doesn't actually give an inspiration for others to raise their morality. So we have to be careful about changing things.
As far as social morality, when we look at the days of the Buddha we see that it was accepted during those times that people can have two or three wives, yet that's not accepted now. So it has changed. We've gotten a little stricter these days. But in other areas we've got a little looser. So far as morality and what is accepted, it's ok as long as it's based in compassion, and not manipulative and abusive. A big difference is that in Theravadin Buddhism we have the Kalama Sutta, which encourages us to investigate for ourselves what is skillful and what is not, what increases greed, hatred and ignorance and what lessens greed, hatred and ignorance.
One of the things that has been very helpful in Theravadin Buddhism in maintaining the tradition and making it last as long as it has, an unbroken lineage from the Buddha, is the strictness of the monks according to these particular rules that can't be changed. So if a Monk is seen to be doing wrong conduct in sexual relations, it is not tolerated, and I believe this is very good. We don't allow them to continue to be in that position of trust, which could then help them to abuse other people as well. So I believe some things need to be kept strict, especially with the Monks. They have to take responsibility for their actions, speech and thoughts, and they will get the results of them if they go against certain things.