In Genesis 2, it is a fore-shadowing, one that requires a lot of explaining. It was originally held using Jewish tradition that this was a reference to the Messiah before the time of Christ, but now that we have the NT, it becomes a bit more clear (I know you probably won't like that as an answer, but it's the best I can do without paragraphs).
In terms of law changing over time, this is only true when you look at the world as a whole. The Law of God hasn't changed, but rather it has been fulfilled. I will explain this in detail below because it's explanation is much shorter.
Again, the OT Law and Prophets point to Christ. Therefore, when we read the OT laws, we should do so looking for Christ--that is to say that the law/Prophets will either point to what Christ would physically do on earth (so we could identify Him when He came), it would point to who He was (as to His nature or attributes), or it would point to His work (namely His mission and the way His mission applies to believers).
Here are some examples:
--I mentioned the sacrifices, but this goes very in-depth. The Law required that for the sin sacrifice, an animal would be an unblemished male--meaning not one that was sickly, diseased or had any defect including broken bones, etc. This points to Christ's perfection--His deity. The animal would be then taken outside of the camp (reflective of the way Christ was killed outside of the city of Jerusalem--apart from His people), and taken to the tabbernacle (meaning the transfer of sin was to sinner would place his hand on the animal as a symbolic transferring of his sin to Christ in the presence of God--a clear depiction of Christ's work on the cross. The animal would then be slaughtered and its blood placed on the altar of the LORD--the altar represents judgement. The law says the animal would be a propitiation (some translation translate it more loosely as 'scapegoat') for the sinner.
--Another example is in the division of clean and unclean foods. I've heard some try to say that this is because God wanted healthy eating, but this interpretation is incorrect because, as mentioned, the OT points to Christ. The division of clean and unclean foods is the division between those that are holy and dedicated to God and those that are not. The Israelites, and we Christians, are not to behave, speak, act, or do things like the world. We are to be different, holy and set apart. This speaks of the cleansing work of Christ when we accept Him as Savior. He sets us apart--calling us out of the things that are unclean, and sinful into holiness. The Law wasn't there to condemn people because of what they ate, but rather it was symbolic of things to come.
--A third thing, one in which you mentioned--the sewing together of two different materials, or the sowing of two different plants into one field is similar to the eating of unclean foods. We are called out of the world--not to be mixed with the world, but to be apart and different. Therefore, the Law could care less about the action, but rather the symbolism behind it.
--Another, and one of my favorites, is the Year of Jubilee. If a man was poor and could not afford to feed or care for his family financially, he could sell himself or his children into service (similar to slavery but without the harsh treatments that come with the connotations of slavery--it more closely matches indentured servitude). Again, if a man could not afford to care for his family, he could sell his possessions--house, land, other properties, etc to pay for debts or raise money to care for his family during his time of need. Once every 50 years, the Israelites were to proclaim a Year of Jubilee at which time slaves would be set free if their debts had not been worked off already, property sold during times of need would be returned to their original owners, and the land would enjoy rest from service. This was to point to Christ's redeeming work of His death on the cross--how He would redeem us from slavery to sin and set us free.
God commanded the Israelites to live in such a way so that their every day living would be symbolic to the work of Christ--that is, it would be a constant reminder of the heart of the issues instead of the letter of the Law. That is why, when Christ came, He fulfilled the Law because He did everything that the Law and Prophets declared He would do. He was a propitiation for our sin, He is what cleanses us from unrighteousness, He is what sets us apart from the world and dedicated to God--bought with a price.
Therefore, much of the Law wasn't there for the purpose of listing strange rules and regulations, but to cause the people to physically live and work out the reminder of what their promised Messiah would one day do.
A vast majority of the Laws were of this type--pointers to Christ. There were some that were set in place to protect people, ensure that people were cared for, and ensure God was honored and glorified. For the rest, they are merely there to depict Christ.