Many people have tried to change Esperanto to their own liking. Have any reform proposals ever entered the general language? (Are there, for example, Ido elements in current usage? – No this is not an attempt to tout reforms, just something I’ve been wondering about! :-) )
A real reform in the sense of replacing language elements by new elements has taken place in 1888, when Zamenhof, after an idea by Edgar von Wahl, in his "Aldono al la Dua Libro" changed the temporal correlatives from -an to -am in order to avoid a collision with the accusative singular of the correlatives ending in -a (kian - kia'n).
From time to time the Esperanto Academy as authorised central institution has sanctioned alternatives to official elements (ekz. the permission to say ark instead of arĥ), but this no real reform, as the older forms remain valid (the Academy is not entitled to remove any official element).
There has been a considerable transfer from Ido to Esperanto, with some elements successfully entering the language (-end-, farbo), other ones still seeking wider recognition (-oz-, -iz-, -iv-). But this also cannot be called a reform, as we're dealing here with an augmentation of the lexicon, not the replacement of any forms.
For some people the preposition far has replaced de with the passive, but this is against the norm (see §6 of the Fundamenta Gramatiko) and not used by the majority of speakers.
Please feel free to correct my English.
If you're asking whether new elements have ever come into Esperanto, then clearly the answer is yes. As just one random example that came up in a recent discussion, the word pigra used to be found only in lists of alternatives to "mal-words." (In this case, as an alternative to maldiligenta.) Now it's an ordinary Esperanto word.