The simple answer is that yes you can replace the come bet. But generally speaking is not to your advantage to do so.
Example, you have a $10 come bet on the 8 with $25 in odds and the shooter makes his point. On the next roll he rolls a 7. You are correct your "base bet" on the 8 is lost ($10) and the odds ($25) are returned. You can replace the $10 come bet and leave your odds up by simply giving the dealer another $10 and telling him to "keep your number up." There is no fee for this. This transaction is called making a "put" bet. Most casinos, but not all, will let you do this as it is to the casino’s advantage. Why? Because when you do this you lose the power of the 7 and 11 when you have a 2 to 1 advantage when you have a bet in the come line. This 2 to 1 advantage is what makes the come bet (or a pass line bet for that matter) one of the best bets on the table. I don’t remember the exact percentage for a put bet, but it is much higher than when you go to a number through the come line.

Hope that helps.

Skinny can give you some advice on this. Search the GTC web site and you will get some good advice on this.

Doc and Rose

The easy way to figure out whether to make a put or place bet is to determine which one would pay more and make the bet that pays more. If they both pay the same, the place bet is a better bet because it is not at risk on a come out seven when you make your point and are on a come out roll again in the future. Whereas the base portion of your put bet can be lost with only the odds being returned on a come out seven.

Say you have $5 on the eight with $25 in odds. As a come bet that would pay $35. But a $30 place bet on the eight also pays $35. So the place bet is a better bet since the entire $30 is not at risk on a come out seven. But with the put bet the $5 base bet would be lost with a come out seven in the future returning only the $25 odds portion of the put bet.