I saw an article that said you can copy your offline D2 classic characters to D2 resurrected. But I haven’t been able to find the answer to if I bought D2 Resurrected, if I could play my battle.net characters I have original D2: LoD online in D2 Resurrected.
This support article explains how you can play your Diablo II (2000) offline single-player characters in Diablo II: Resurrected; it also lists the limitations: