There are differences between texts of Tanach now and in the past. In fact, there are several different so-called "textual witnesses" of Tanach that exist, each of which has their own strengths and weaknesses.
The textus receptus — received text of the Jews is the Masoretic text, whose ancestors are present in some form in some of the Dead Sea Scrolls and other books found in the Judean desert. It is clear linguistically and archaeologically that the Masoretic text is old, but not the urtext of Tanach.
Other traditions, such as those in the Samaritan Pentateuch, in the LXX and in other non-premasoretic scrolls have certain elements that appear to be more acceptable than those in the textus receptus.
Most of the changes are rather insignificant, e.g. chaser vs. maleh, additional "ve-", confusion of letters in transcription. Other changes are more substantial, like random ommissions:
Masoretic: וַיֹּאמֶר קַיִן, אֶל-הֶבֶל אָחִיו... וַיְהִי בִּהְיוֹתָם בַּשָּׂדֶה
Samaritan (~LXX, Peshita, etc.): ויאמר קין אל הבל אחיו נלכה השדה ויהי בהיותם בשדה
or even some apparently intentional changes, such as this anti-polytheistic one
Masoretic (=Samaritan, Peshitta, etc.): בְּנֵי יִשְׂרָאֵל
4QDeutj: בני אלהים
Within the Masoretic text itself, there are variations. These are often regarding small details, like placement of ga``aot (metegs) or te`amim.
Nonetheless, despite the differences between different manuscripts are small and seldom change the meaning of things. Certainly the text has not been corrupted remotely close to the extent claimed.
M. Goshen-Gottstein. The Book of Isaiah, sample edition with Introduction.
E. Tov. Textual Criticism of the Hebrew Bible.
I. Yeivin. Introduction to the Tiberian Masorah.