Warmup (you've probably seen this before)
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Suppose $\sum\_{n\ge 1} a\_n$ is a conditionally convergent series of real numbers, then by rearranging the terms, you can make "the same series" converge to any real number $x$. To do this, let $P=\{n\ge 1\mid a\_n\ge 0\}$ and $N=\{n\ge 1\mid a\_n<0\}$. Since $\sum\_{n\ge 1} a\_n$ converges conditionally, each of $\sum\_{n\in P}a\_n$ and $\sum\_{n\in N}a\_n$ diverge and $\lim a\_n=0$.


Starting with the empty sum (namely zero), build the rearrangement inductively. Suppose $\sum\_{i=1}^m a\_{n\_i}=x\_m$ is the (inductively constructed) $m$-th partial sum of the rearrangement. If $x\_m\le x$, take $n\_{m+1}$ to be the smallest element of $P$ which hasn't already been used. If $x\_m> x$, take $n\_{m+1}$ to be the smallest element of $N$ which hasn't already been used.


Since $\sum\_{n\in P}a\_n$ diverges, there will be infinitely many $m$ for which $x\_m\ge x$, so $n\_{m+1}$ will be in $N$ infinitely often. Similarly, $n\_{m+1}$ will be in $P$ infinitely often, so we've really constructed a rearrangement of the original series. Note that $|x-x\_m|\le \max\{|a\_n|\bigm| n\not\in\{n\_1,\dots, n\_m\}\}$, so $\lim x\_m=x$ because $\lim a\_n=0$.




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> Suppose $\sum\_{n\ge 1}v\_n$ is a conditionally convergent series with $v\_n\in \mathbb R^k$. Can the sum be rearranged to converge to any given $w\in \mathbb R^k$?
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> 
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Obviously not! If $\lambda$ is a linear functional on $\mathbb R^k$ such that $\sum \lambda(v\_n)$ converges absolutely, then $\lambda$ applied to any rearrangement will be equal to $\sum \lambda(v\_n)$. So let's also suppose that $\sum \lambda(v\_n)$ is conditionally convergent for every non-zero linear functional $\lambda$. Under this additional hypothesis, I'm pretty sure the answer should be "yes".



Can a conditionally convergent series of vectors be rearranged to give any limit?
The Levy--Steinitz theorem says the set of all convergent rearrangements of a series of vectors, if nonempty, is an affine subspace of ${\mathbf R}^k$. There is an article on this by Peter Rosenthal in the Amer. Math. Monthly from 1987, called "The Remarkable Theorem of Levy and Steinitz". Also see Remmert's Theory of Complex Functions, pp. 30--31.


As an example, taking $k = 2$, suppose $v\_n = ((-1)^{n-1}/n,(-1)^{n-1}/n)$. Then the convergent rearrangments fill up the line $y = x$. The linear function $\lambda(x,y) = x-y$ of course kills the series, which makes Anton's observation explicit in this instance.


The Rosenthal article, at the end, discusses Anton's question. Indeed if there is no absolute convergence in any direction then the set of all rearranged series is 
all of ${\mathbf R}^k$. Note by the above example that this condition is stronger than saying the series in each standard coordinate is conditionally convergent. Rosenthal said this stronger form of the Levy-Steinitz theorem was in the papers by Levy (1905) and Steinitz (1913). He also refers to I. Halperin, Sums of a Series Permitting Rearrangements, C. R. Math Rep. Acad. Sci. Canada VIII (1986), 87--102. 


