She is in a safeguard and safeguard go so she is used to the harness. She used to be 63 pounds until last month then she grew overnight like two inches, gained like 12 pounds and became size 10 in clothing. That's why I'm now looking for something appropriate for her size! And she's my baby, her brother is in college!
If you want to be able to harness for a little longer, and she'll fit in the Britax Frontier 85 longer in harnessed mode, I don't think there's anything wrong with buying it. :twocents: It's a great seat, and it should last her a long time in booster mode when it's time to use it in booster mode. I think she probably should have a least a little room in harnessed mode in a Frontier 85, especially since she still fits in a Safeguard Go with her shoulders under the top harness position. I harnessed my daughter all the way through first grade, but I think at your daughter's age, a booster would also be a good choice.
Right now though, as I'm sure you know, she is over the harnessed limit for the Safeguard Go (I believe the limit on that version was 60 pounds) as well as the Safeguard (I believe the limit is 65 pounds), so she needs a new seat before riding in those again, unless I am mistaken about the limits. I know that the Go only becomes a backless booster, and I don't usually advise going straight to a backless booster from a harness, but I'd convert the Go to booster mode until you are able to buy a new seat (whether the Frontier 85 or a booster) to use instead of using her current seats in harnessed mode.
Good luck seat shopping! I love the Frontier 85 (I have 3 of them!), so I don't think it's a bad choice, especially if you can get it at a store and start using it right away, but a booster is also a great choice for a first grader!