If your child is only year 1 then it's a while,at least year 3/4 before you will get a decent idea as to whether they could pass grammar entry and whether it's the right fit for them, and even if they will want to go there. A bright 5 year olds academic profile may look very different at 9/10 years old, and the brightest at year 1 in DD's now year 5 class are now very much middle of the class academically, not the top 10% needed for grammar. Others who have been less pressured have taken the top spots. The 11+ process is stressful and places strain on the family, most 9 year olds want to be playing with their mates rather than doing tutoring.
We live in a grammar area (Bucks), moving here when DD's were 4 and 1, so secondaries weren't something we were thinking about. If I had known more, I think we may not have moved here. DH and I, and both our girls are very able academically, but we have concerns about her going to the all girls grammar she has her heart set on. It's a fantastic school, with great facilities and great results, but we don't feel its the right fit for her having visited and knowing many families with girls there. Part of me hopes she doesn't pass the 11+, so the decision is taken away, as it would be hard for us to tell her no if she does pass.
Schools will also change in this time becoming better or worse.
But to answer your question - yes, would need to be living in catchment by start of year 6. But would suggest moving start year 5, to get a feel for the local schools, and so they can settle in before taking the test. The tutoring you will likely need will start from beginning of year 5, and tutors local to area would be much better at knowing what's needed
Be very careful how you frame this though, telling a child you are moving to get them into grammar places way too much pressure on them. It's needs to be move for the family as a whole rather than just for a school.