you can try to add the scheme code below to your init-texmacs.scm (extracted from the current svn).
It seems to work for me, but I'm not sure if it will work in your version.
But maybe then you will have just to tweak a little. Then the shortcut Joris implements will be available to you until you can install a more recent version.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Switching between displayed equations and equation arrays
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(define (search-labels t)
(tree-search t (lambda (l) (tree-in? l (label-tag-list)))))
(define (cut-all t w)
(cond ((tree-atomic? t) (noop))
((tree-is? t w) (tree-cut t))
(else (for-each (cut cut-all <> w) (tree-children t)))))
(define (check-border? l empty?)
(cond ((null? l) #t)
((and (not empty?) (nnull? (car l)) (!= (caar l) "")) #f)
(else (check-border? (cdr l) (== (cAr (car l)) "")))))
(define (convertible-eqnarray? t)
(let* ((rs (select (tm->stree t) '(:* row)))
(rcs (map (lambda (r) (select r '(:* cell 0))) rs)))
(check-border? rcs #t)))
(tm-define (eqnarray->equation t)
(with labs (map tree-copy (search-labels t))
(when (and (<= (length labs) 1) (convertible-eqnarray? t))
(cut-all t 'label)
(cut-all t 'eq-number)
(let* ((l* (select t '(:* cell 0)))
(l (if (null? labs) l* (cons (car labs) l*)))
(c (apply tmconcat (map tree->stree l)))
(n (if (null? labs) `(equation* ,c) `(equation ,c))))
(tree-set! t n)
(tree-go-to t 0 :end)))))
(define (binary-relations)
(or (get-packrat-definition "std-symbols" "Relation-nolim-symbol") (list)))
(define (binary-relation? t)
(in? (tm->stree t) (binary-relations)))
(define (atom-decompose t)
(if (tree-atomic? t)
(tmstring->list (tree->string t))
(list t)))
(define (concat-decompose t)
(cond ((tree-atomic? t) (atom-decompose t))
((tree-is? t 'concat)
(apply append (map atom-decompose (tree-children t))))
(else (list t))))
(define (make-eqn-row-sub l)
(list "" (car l) (apply tmconcat (cdr l))))
(define (finalize-row l)
`(row (cell ,(car l)) (cell ,(cadr l)) (cell ,(caddr l))))
(tm-define (equation->eqnarray t)
(let* ((labs (map tm->stree (search-labels t)))
(c* (tree-ref t 0))
(c (if (tree-func? c* 'document 1) (tree-ref c* 0) c*))
(l0 (concat-decompose c))
(l1 (list-filter l0 (lambda (x) (not (tm-is? x 'label)))))
(l2 (list-scatter l1 binary-relation? #t)))
(when (>= (length l2) 2)
(cut-all t 'label)
(let* ((l3 (cons (list (apply tmconcat (car l2))
(caadr l2)
(apply tmconcat (cdadr l2)))
(map make-eqn-row-sub (cddr l2))))
(l4 (if (null? labs) l3
(rcons (cDr l3)
(rcons (cDr (cAr l3))
(tmconcat (cAr (cAr l3))
'(eq-number) (car labs))))))
(l5 (map finalize-row l4))
(r `(eqnarray* (document (tformat (table ,@l5))))))
(tree-set! t r)
(tree-go-to t 0 0 0 :last :last 0 :end)))))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Special toggles
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(kbd-map
(:require (inside? 'equation*))
("C-&" (equation->eqnarray (tree-innermost 'equation*))))
(kbd-map
(:require (inside? 'equation))
("C-&" (equation->eqnarray (tree-innermost 'equation))))
(kbd-map
(:require (inside? 'eqnarray*))
("C-&" (eqnarray->equation (tree-innermost 'eqnarray*))))