This is the only instance I know of that רעואל is known as the father of Eliasaph , דעואל is known as the father of Eliasaph in bemidbar 1.14, and 3 other places, the resh and the Dalet are very similar, is that the reason ?
There are a number of possibilities as shown by Rabbi Buchwald. Rav Hirsch, besides citing the Ramban refers to his own commentary on Vayishlach 36:1 which explains the concept of multiple names for individuals.
Note that THE BIBLE "CODES": A TEXTUAL PERSPECTIVE
- The two versions are laid out side by side, with differences highlighted, in Abba Bendavid, Makbilot BaMikra' (Parallels in the Bible) (Jerusalem: Carta, 1972), p. 61-62. Kimhi held that such differences as Dodanim in Gen. 10:4 vs. Rodanim in 1 Chron. 1:7, and Deuel in Num. 1:14 vs. Reuel in 2:14 are due to confusion of similar letters, but he held that the confusion took place in pre-Biblical texts and that the Bible intentionally preserved both forms to show that they referred to the same peoples or persons; there was no confusion in the transmission of the Bible itself (see his comments to Gen. 10:4 and 1 Chron. 1:7, and Uriel Simon, "Ibn Ezra and Kimhi -- Two Approaches to the Question of the Accuracy of the Masoretic Text," Bar Ilan 6 (1968):208-209).
Points out that the Radak mentioned below is not from Bamidbar but is referred to as part of his explanation of this and similar differences. I have not found the actual location in Radak so I do not know (other than the quoted article), where the Radak says this.
I found a reference to Moshe Cassuto, professor of Bible at the Hebrew University, who suggests an alternative answer. He suggests (involving the Dodanim in Noach 10:4 and Divrei Hayamim I 1:7 that in both Dodanim and Deuel, the names are actually shortened forms of a longer name.
Once again we have a “daled” and a “resh” interchanged. He suggests that the original name of the people was “Derodanim”, however, they were known better in its shorter form, as both the Dodanim and the Rodanim. So too, Eliasaph's father. His full name was Deru’el. However, he was called by both its shorter forms; sometimes called De’uel and sometimes Re’uel.
The commentators raise an issue regarding the leader’s name. In the very next chapter, in Numbers 2:14, when the camp’s setup and structure are described, the prince of the tribe of Gad is identified with a slight change as, אֶלְיָסָף בֶּן רְעוּאֵל, Eliasaph the son of Reuel, not “Deuel.”
Nachmanides suggests that Eliasaph’s father had two names, “Deuel,” which indicates that he knew G-d, and “Reuel,” indicating that he constantly imagined G-d in his heart. Scripture preserved both names in order to convey that both these special qualities were found in Eliasaph’s father.
The Radak says that both names are actually identical. He attributes the change to the fact that both the Hebrew letters, ד–“dalet” and ר–“raysh,” are graphically similar, and are consequently often interchanged. Therefore, some people pronounce the name דְּעוּאֵל–“Deuel” while others pronounce it רְעוּאֵל–“Reuel.” The Torah preserves both names in order to underscore that both names are essentially the same.
Some commentators identify “Reuel” as Jethro, Moses’ father-in-law. After all, Reuel (Exodus 2:18) was one of Jethro’s seven names. The Baalei Tosafot contend that after he converted, Jethro’s name was formally changed to “Deuel,” indicating that Jethro knew G-d. The problem with this interpretation is that it fails to explain why the child of a convert is listed as the leader of the tribe of Gad.
Note that the child of a (male) convert does not belong to any tribe. However, this may be a reference showing that someone named Reuel could also be called Deuel and not that he was actually the son of Yisro (Reuel - Deuel)
The Imrei Noam , cited by Pninim ahl HaTorah, says that the change of names comes to teach an important ethical lesson. The Midrash states that the tribe of Dan, who was the firstborn child of Zilpah (Leah’s handmaiden), was given a great honor and was designated to lead an entire דֶּגֶל–degel (banner), that included the tribes of Asher and Naphtali. Gad, who was the first born child of Bilhah (Rachel’s handmaiden), after all, could have easily protested why Dan was given the honor of leading a banner of three tribes and not Gad. Therefore, because Eliasaph was prepared to concede and forego the deserved honor, and did not complain, his father’s name was changed to “Reuel,” which means, רֵעַ אֵ־ל–“Ray’ah Kayl,” a friend of G-d, just like Moses. One who avoids disputes, and is willing to forego a truly deserved honor, is considered to be a true friend of G-d. Additionally, although the exact place of Moses’ burial is not known, he is buried in the territory of Gad, on the east bank of the Jordan.