This decision is based on several things, not just the age of that one card.
How old are your other accounts? [You AAoA takes a smaller hit if you accept but have older accounts]
How many other accounts do you have? [AAoA overall is less sensitive if you have multiple accounts, especially if the others are older than this one]
How many other Visa/MC/Discover/Amex cards do you have? [Will this help you out, if you can use it anywhere?]
Do you frequently use this card at BP because you generally use BP for gas, or would you use the card more if you had the ability to use it anywhere? [Since in some cases usage can lead to faster/better CLI, if you intend to grow this card and keep it long-term, how will your usage compare if it remains BP-only?]
Is there an improved rewards schedule with the Visa version?
Is there a better or worse APR with the Visa version?
Is there an annual fee with the Visa version?
The shift from one to the other (closing a <year old card for a new one) may affect your AAoA, but if your file is quite young/thin, this is not a priority consideration.
In one year, the new card will be a year old. I know that can sound like a long time, but in the realm of building/rebuilding credit, a year is just a year. A brick-solid, healthy, top-tier file generally has much more life on it than a year, and so this one-year/two-year consideration in your situation is really not something to weigh too heavily. Other factors - including usage ability, ability to grow the account, cost of interest/AF, etc. - should probably be given more weight.
