This is extremely common in 2026 and usually fine, but it's worth a quick check rather than a shrug. Panel supply has been volatile, so being quoted one premium mono panel and offered an equivalent close to install date is routine — the most frequent swap right now is between the big three tier-1 brands (Aiko, Longi, Jinko), which are genuinely comparable performers. A like-for-like substitution at the same wattage, same all-black mono N-type spec, and same or better warranty is not a downgrade and not something to panic over.
What you do want to confirm: that the replacement is the same wattage or higher (so your total kWp doesn't quietly shrink), the same tier-1 class with an equal or better product warranty, and the same physical fit (some panels are a few centimetres taller, which can matter on a tight roof). Get the swap and the (unchanged) price confirmed in writing. The substitutions to push back on are a drop to an unknown generic brand, a quietly lower wattage, or fewer panels — those change what you're paying for.
If the swap is one tier-1 mono panel for another at equal spec, accept it and move on; the difference in real-world output between, say, Aiko and Longi at the same wattage is marginal. If it's a downgrade in brand class, wattage or count, treat it as a renegotiation and get the price adjusted or the original honoured.