If the teshuva is genuine, not only is it regarded as being a proper teshuva, but in some instances, it is regarded as being even more praiseworthy as the person has changed the habits of a lifetime to become a better Jew.
In fact, there is a story that is related that helps to bring this idea out.
Yosi ben Yoezer Ish Tzreida (יוסי בן יועזר איש צרדה) - as mentioned in Pirkei Avos 1:4 lived during the Greek era and was being led to his execution. His nephew Yokim Ish Tzeruros (יקים איש צרורות) was a Jewish renegade who switched to the Greek side, and he went to watch his uncle's impending death. Seeing this as a good opportunity to goad his uncle and prove that he had made the right life decisions, he sneered, "Look at you now!". Gazing at his nephew, Yosi ben Yoezer responded swiftly, noting, "If this is the reward for a רשע (an evil person) (i.e. his nephew was riding on a stallion), how much greater is the reward for a צדיק (a righteous person) in the next world!". יקים איש צרורות countered, "Is there a greater tzaddik than you?!" to which יוסי בן יועזר answered, "If this is the punishment (i.e. a great torture) for a צדיק, how much greater is the punishment for a רשע!"
This exchange had a profound impact on יקים איש צרורות who reasoned that due to his ill behaviour he was guilty of the death penalty according to halacha. He consequently built a contraption that would kill him simultaneously with all four of the death penalty methods.
This served as such a כפרה (atonement) that a moment before יוסי בן יועזר was killed he saw a vision of his nephew and he related that my nephew entered עולם הבא (the Next World) one moment before me.
(Sources: See Bereishis Rabbah 65:22 - "נִתְנַמְנֵם יוֹסֵי בֶּן יוֹעֶזֶר אִישׁ צְרֵידָה וְרָאָה מִטָּתוֹ פָּרְחָה בָּאֲוִיר, אָמַר בְּשָׁעָה קַלָּה קְדָמַנִּי זֶה לְגַן עֵדֶן". Also related by the Rishon Rabbi Yisrael ben Yosef Alnaqua in his Menoras HaMaor. Refer as well to this modern commentary here.)
Indeed, on this note refer to Pirkei Avos 4:17:
הוּא הָיָה אוֹמֵר, יָפָה שָׁעָה אַחַת בִּתְשׁוּבָה וּמַעֲשִׂים טוֹבִים בָּעוֹלָם הַזֶּה, מִכָּל חַיֵּי הָעוֹלָם הַבָּא. וְיָפָה שָׁעָה אַחַת שֶׁל קוֹרַת רוּחַ בָּעוֹלָם הַבָּא, מִכָּל חַיֵּי הָעוֹלָם הַזֶּה
He used to say: more precious is one hour in repentance and good deeds in this world, than all the life of the world to come; And more precious is one hour of the tranquility of the world to come, than all the life of this world.