For different circumstances you would use different forms. Here are some instances.
• “Come to my room”: While away from your room, you tell a person to come with you to your room; or via telephone, tell someone to come to your room.
• “Come in my room”: Standing outside the door of your room talking with someone, you ask them to come in to continue a discussion. But typically it would be stated more briefly or at least differently: “Come in”, “Come on in, let's sit down”, etc without mentioning the room, since it is right there anyway. Note, into is a likely prospect when the room is mentioned.
• “When I came in the lobby in the evening, there were tons of people gathered around...”: This form is inappropriate; use into instead. I'd use into for going into a lobby from inside a hotel.
• “When I came to the lobby in the evening, there were tons of people gathered around...”: I'd use to for going to a lobby in a hotel, from elsewhere, but typically no real distinction will be made between use of to and into in this context.
• “When I came to in the lobby in the evening, there were tons of people gathered around...”: This form is appropriate if you passed out and are regaining consciousness.
If you ask how long ago, you would get the answers with a number and a time unit, like one year, 3 weeks, etc.
If you ask when, you would get a specific time / date.
Meanwhile you can use when to ask events happen in future.
Best Answer
I would say that came up to means roughly the same as approached (that is, move into close proximity with):
whereas came to can be used to mean arrive at:
Obviously, there is some overlap between the two, because when you move close to something, you are also arriving there. Because of that, I would say that you could always use came to, but you wouldn't necessarily always be able to always use came up to without a possible slight shift in meaning. For example, in my second sentence, if I were to say:
that might imply something different. It could imply the doctor walked up along the road and stood at the fence (as opposed to in the original sentence, where it's easier for me to imagine the veterinarian in the kitchen). Another possible interpretation after adding the word up is that the veterinarian came from further away – in other words: I remember when the veterinarian came up [from the city] to bring us the bad news.
However, if I said:
that pretty much says the same as the original, because we approach a ticket counter in the same way we arrive at a ticket counter.
As a footnote, these usages don't include more idiomatic uses of came to, such as: