Then you use the plain form of the adjective. If the temperature is already high, then you could say, "If I stay where I am, it will remain hot." If the temperature is already low, you would say, "It will remain cold." Likewise you could use whatever word is appropriate for the current temperature: warm, cool, broiling, freezing, lukewarm, comfortable, whatever.
I don't know a single word that means "the temperature is the same as it was before". You'd have to use several words for that, like, "The temperature will be unchanged" or "it will remain the same temperature". If it's obvious from context that you're talking about temperature, then you could just say "it will be the same" or "unchanged" or "constant", etc.
Similar things could be said about other comparative words. "Al is taller than Bob" or "Al is shorter than Bob", but there's no single word I can think of that you could fill into "Al is X than Bob" that means "is the same height". Normally we'd say "Al is the same height as Bob" or something similar.