Sense switch is appropriate, I think. All good points about 来る/来たる.

It is interesting however notice when you check common usages as adj 来たる has more hits:

It seems the notation 来たるべき is quite established 
"来たるべき" -> 4,980,000
"来るべき" -> 2,290,000

In other cases 来る surpasses 来たる by an average 40%
"来る者" -> 974,000
"来たる者"　-> 617,000

"来る春" -> 74,800
"来たる春"-> 43,700

Except for dates, where 来る clearly wins over 来たる being from 5 to 10 times more used.
"来る4月" -> 1,250,000
"来たる4月"　-> 233,000

"来る25日" -> 156,000
"来たる25日"　-> 15,500 

I suppose the reason is quite intuitive: when you have dates it is very easy to understand 来る is meant to be read きたる. It is a shorter notation, very practical. However in other cases even Japanese people get confused. When they see 来る者 for instance, they will need to think how to read it, while 来たる here avoids ambiguity. That is the same reason 来たるべき　with the time became such an established notation. It is not only us to have problems reading kanji, Japanese people go though the same hardships.

sorry.  got that comment backwards.  some dics suggest when a verb 来たる; when an adj-pn 来る

daijr 3rd ed has 来たる as well now, but only in its entry for the verb.  daijr's entry for adj-pn is strictly 来る.
=====
meikyo:
きた・る【来たる】
[自四]〔古〕くる。やってくる。
「○○先生、━」
「待ち人━・らず」
*「来至る」の転という。文語動詞であるため、常用漢字表の例欄には語例を掲げないが、送りがなは「来くる」に準じて「来たる」と送る（ただし、許容では「来る」とできよう）。
-
きたる【来る】
[連体]《日付などの上に付いて》もうすぐくる。この次にくる。
「━五日は創立記念日だ」
*動詞「来たる」の連体形から出た語だが、慣用に従って「来る」と送る。動詞の場合とは送りがなが異なる。

-some dictionaries (e.g., daijr, meikyo) suggest that when a verb it should be written as 来る, whereas when adj-pn it should be written as 来たる.
-i don't think an ichi tag is out of line for this entry. sense 1 is old-fashioned but sense 2 is not uncommon.  i suspect that the ichi tag legitimately belongs on 来る, though i have no evidence.

@@ -14,7 +13,0 @@
-<pos>&v5r;</pos>
-<pos>&vi;</pos>
-<gloss>to come</gloss>
-<gloss>to arrive</gloss>
-<gloss>to be due to</gloss>
-</sense>
-<sense>
@@ -26,0 +20,8 @@
+<sense>
+<pos>&v5r;</pos>
+<pos>&vi;</pos>
+<s_inf>orig. meaning</s_inf>
+<gloss>to come</gloss>
+<gloss>to arrive</gloss>
+<gloss>to be due to</gloss>
+</sense>

GG5, 中辞典, etc. (both have 来たる as headword)

(a) Yes, the "Ichiman" wordlist does have 来る/きたる on it.
(b) I see two modern refs are now using 来たる, so the "io" should come off it.
(c) It might be prudent to put the "ichi1" tag on 来たる instead, in which case it should go first?
(d) I have no idea which of 来たる/きたる and 来る/きたる is more common. It's difficult to test.

@@ -6 +5,0 @@
-<ke_pri>ichi1</ke_pri>
@@ -10 +8,0 @@
-<ke_inf>&io;</ke_inf>
@@ -14 +11,0 @@
-<re_pri>ichi1</re_pri>