You are allowed to overbuild your own Industry tiles with any higher-level tile of the same industry type during both the Canal Era and the Rail Era, without restriction.

This is supported by the following rules:

• On page 10, under Overbuilding, it states:
  • "If the tile you are replacing is your own: You may Overbuild any Industry tile."
• There is no mention of era-based restrictions or limitations based on tile level for overbuilding your own tiles.
• The only restrictions mentioned for overbuilding apply when overbuilding opponents’ tiles (Coal Mines/Iron Works, and only when there are no resource cubes left on any Industry tiles or in the Market).

Source: page 10

Summary: You may overbuild your own Industry tiles with any higher-level tile of the same type at any time, in either era, with no additional restrictions.

Source: Core Rulebook (pp. 10, 12, 8, 9)