We are still waiting for the official confirmation of the news that remasters of old GTA games (GTA 3, Vice City, and San Andreas) are in development. But that's pretty likely because Take-Two and Rockstar Games have been fighting the unofficial "remasters" of those GTA games for the past month.
For some time now, there have been mods that insert city maps from previous GTA games into GTA 5. Now such mods have been banned from distribution because the owners of the GTA series do not want to be put in a situation where someone confuses players/customers about who are the real and who are the wrong remasters.
An example of one such mode was demonstrated a month ago with “maximum graphics”. If you watch the video it will be clear to you why the owners of the GTA franchise are fighting against mods like this. As advanced as they may seem at first, they could create the wrong vision of an official remaster that will reportedly not run on the same engine as GTA 5 but Unreal Engine.
The example of the above mode is also a good indicator of how the mode for GTA 5 can insert an old map into the game, but the visual atmosphere itself is significantly different from that in the original game.