Check out the article by this name by Ian Young, Robert Rezetko, and Martin Ehrensvärd at The Bible and Interpretation. The bottom line: rather than talking about “Early Biblical Hebrew” and “Late Biblical Hebrew,” we should be talking about “more conservative” and “less conservative” literary styles, both of which were available to scribes in both pre-exilic and post-exilic times. Furthermore, biblical books (like all other ancient documents) were subject to textual emendation to update the spelling and language over the centuries:
Thus the evidence in our possession indicates a high degree of fluidity of biblical texts in the BCE period. This fluidity is especially noticeable in regard to the language of the biblical texts. At the very least, individual linguistic elements came and went during scribal transmission. The text-critical evidence, therefore, puts a question mark over the whole enterprise of linguistic dating before it has begun, since linguistic dating could only work if the language of the current texts is very close, if not identical to the language of the “original author” of the text being dated. On the contrary, the text-critical evidence indicates that the current linguistic profiles of the biblical books are not only the result of choices made by their authors only but also by later scribes.
Therefore, we need to be clear about what we think we’re actually “dating” when determining the date of a biblical book:
If biblical books were written and rewritten over the generations in the BCE period, then the question of the “original date” when a biblical book was composed is revealed to be anachronistic and irrelevant. The book as a whole was composed over a long period of time. Beyond this there are various things we may try to date such as when do we think the core form of the book came into being or what is the date of the current form of the text we are studying? We can see that these are two separate questions and that we must be very cautious about using the features of the current texts to date a presumed original composition.
There may be a number of valid reasons to attempt to date the “core form” of a biblical book, and Young et al. lay a groundwork for concluding that this “core form” may have arisen quite early despite seemingly “late” linguistic features. (Of course, the opposite is also possible: a seemingly early book may in fact be late.). But whenever you hear someone arguing for a biblical book’s “lateness” or “earliness” based on linguistic considerations alone, it might be wise to turn off your hearing aid and smile politely until such time as you can evaluate those claims based on other lines of evidence.