True, I actually just took a look at the rule book and if you go from one eligible position to another you don't have to change. So disregard my first post, I stand corrected.
ARTICLE 2. PLAYERS NUMBERED BY POSITION. All players must wear numerals on their jerseys in accordance with Rule 5, Section 4, Article 3, Item 3. Such numerals must be by playing position, as follows:
(a) quarterbacks, punters, and placekickers: 1–19;
(b) running backs and defensive backs: 20–49;
(c) centers: 50–79;
(d) offensive guards and tackles: 60–79;
(e) wide receivers: 10–19 and 80–89;
(f) tight ends and H-backs: 40–49 and 80–89;
(g) defensive linemen: 50–79 and 90–99;
(h) linebackers: 40–59 and 90–99.
If a player changes his position during his playing career in the NFL, and such change moves him from a position as an ineligible pass receiver to that of an eligible pass receiver, or from a position as an eligible pass receiver to that of an ineligible pass receiver, he must be issued an appropriate new jersey numeral. A change in jersey numeral is not required if the change is from an ineligible position to another ineligible position, or from an eligible position to another eligible position, provided that the player has participated at least one season at his position prior to the change.
Any request to wear a numeral for a special position not specified above (e.g., H-back) must be made to the Commissioner. During the preseason period when playing rosters are larger, the League will allow duplication and other temporary deviations from the numbering scheme specified above, but the rule must be adhered to for all players during the regular season and postseason. Clubs must make numerals available to adhere to the rule, even if it requires returning to circulation a numeral that has been retired or withheld for other reasons.